feat: folder CRUD with discard-or-delete dialog

The left sidebar can now create, rename, and delete folders. Each
operation is mirrored to disk through the backend.

Backend (folders router):
- POST /folders { name, parent_id } — create a sub-folder under an
  existing Folder row, mkdir on disk, insert the row, return it. Names
  are validated (no separators, no traversal).
- PATCH /folders/{id} extended — still does the display-only rename for
  SourceRoot ids, but for Folder ids it now actually moves the directory
  on disk and rewrites every descendant Folder.path + Photo.filepath
  that lived under the old prefix in a single transaction. Refuses to
  rename the source-root mount itself.
- DELETE /folders/{id}?mode=discard|permanent —
    discard: set is_discarded on every photo whose filepath lives under
             this folder. The folder, descendants, and on-disk dir are
             left intact. Recoverable from the discard pile.
    permanent: unlink each file, remove rows, rmtree the directory.
- Refuses to delete the source-root mount in either mode.

Frontend:
- New DeleteFolderDialog: two-card mode picker (Move to discard pile /
  Permanently delete) with destructive accent on the latter. Esc and
  backdrop click cancel.
- LeftSidebar: hover-revealed kebab menu on every folder row with
  New sub-folder, Rename, and Delete folder…  Inline create input
  appears below the parent row when "New sub-folder" is picked.
  All mutations invalidate ['folders'], ['photos'], and the library
  stats query so the sidebar counts stay live.
- api.ts: sourceFolders.create + sourceFolders.delete wrappers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/LeftSidebar.tsx

@@ -1,8 +1,9 @@
-import { useState } from 'react'
+import { useEffect, useRef, useState } from 'react'
 import {
   ChevronRight,
   ChevronDown,
   Folder,
+  FolderPlus,
   Image,
   Star,
   Trash2,
@@ -11,6 +12,8 @@ import {
   Copy,
   Tag as TagIcon,
   Layers2,
+  MoreHorizontal,
+  Pencil,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { sourceFolders, library, photos as photosApi, type FolderTreeNode } from '../../services/api'
@@ -27,6 +30,7 @@ import {
 } from '../../hooks/useLibraryStatsQuery'
 import { registerUndoable } from '../../store/undoStore'
 import type { Photo } from '../../types/photo'
+import { DeleteFolderDialog } from '../dialogs/DeleteFolderDialog'
 
 interface TreeItem {
   id: string
@@ -52,6 +56,40 @@ export function LeftSidebar() {
   const { data: stats } = useLibraryStatsQuery()
   const [dropTargetId, setDropTargetId] = useState<string | null>(null)
 
+  // Per-folder kebab menu open state. Stores the tree-item id ("folder-..."
+  // or "folders" for the section header). Outside-click + Escape close.
+  const [openMenuId, setOpenMenuId] = useState<string | null>(null)
+  const menuRef = useRef<HTMLDivElement>(null)
+  useEffect(() => {
+    if (!openMenuId) return
+    const onDown = (e: MouseEvent) => {
+      if (menuRef.current && !menuRef.current.contains(e.target as Node)) {
+        setOpenMenuId(null)
+      }
+    }
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') setOpenMenuId(null)
+    }
+    document.addEventListener('mousedown', onDown)
+    document.addEventListener('keydown', onKey)
+    return () => {
+      document.removeEventListener('mousedown', onDown)
+      document.removeEventListener('keydown', onKey)
+    }
+  }, [openMenuId])
+
+  // "Create new folder under {parent}" inline state. parentId is the
+  // Folder.id (no "folder-" prefix).
+  const [creatingUnder, setCreatingUnder] = useState<string | null>(null)
+  const [createDraft, setCreateDraft] = useState('')
+
+  // Folder being deleted, drives the DeleteFolderDialog mounted below.
+  const [deletingFolder, setDeletingFolder] = useState<{
+    id: string
+    name: string
+    photoCount?: number
+  } | null>(null)
+
   // Bulk discard mutation for the drag-onto-Discarded interaction.
   const discardDropMutation = useMutation({
     mutationFn: (photoIds: string[]) => photosApi.bulkDiscard(photoIds),
@@ -217,11 +255,55 @@ export function LeftSidebar() {
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['folders'] })
       queryClient.invalidateQueries({ queryKey: ['folders', 'tree'] })
+      queryClient.invalidateQueries({ queryKey: ['photos'] })
     },
     onError: (e: any) =>
       toast.error('Rename failed', e?.response?.data?.detail || e.message || 'Unknown error'),
   })
 
+  const createFolderMutation = useMutation({
+    mutationFn: ({ parentId, name }: { parentId: string; name: string }) =>
+      sourceFolders.create(parentId, name),
+    onSuccess: (data) => {
+      toast.success('Folder created', data.name)
+      queryClient.invalidateQueries({ queryKey: ['folders'] })
+      queryClient.invalidateQueries({ queryKey: ['folders', 'tree'] })
+      setCreatingUnder(null)
+      setCreateDraft('')
+    },
+    onError: (e: any) =>
+      toast.error('Create failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+  })
+
+  const deleteFolderMutation = useMutation({
+    mutationFn: ({ id, mode }: { id: string; mode: 'discard' | 'permanent' }) =>
+      sourceFolders.delete(id, mode),
+    onSuccess: (data) => {
+      if (data.mode === 'discard') {
+        toast.success(
+          'Folder photos discarded',
+          `${data.discarded ?? 0} moved to discard pile`
+        )
+      } else {
+        toast.success(
+          'Folder deleted',
+          `${data.deleted_photos ?? 0} photos removed from disk`
+        )
+      }
+      queryClient.invalidateQueries({ queryKey: ['folders'] })
+      queryClient.invalidateQueries({ queryKey: ['folders', 'tree'] })
+      queryClient.invalidateQueries({ queryKey: ['photos'] })
+      queryClient.invalidateQueries({ queryKey: LIBRARY_STATS_QUERY_KEY })
+      // If we were viewing the deleted folder, snap back to all-photos.
+      if (deletingFolder && currentSection === `folder-${deletingFolder.id}`) {
+        navigateToSection('all-photos', {})
+      }
+      setDeletingFolder(null)
+    },
+    onError: (e: any) =>
+      toast.error('Delete failed', e?.response?.data?.detail || e.message || 'Unknown error'),
+  })
+
   // Mutation for scanning all folders
   const scanLibraryMutation = useMutation({
     mutationFn: library.scan,
@@ -455,8 +537,127 @@ export function LeftSidebar() {
             <span className="h-5 min-w-[24px] flex-shrink-0" aria-hidden="true" />
           )}
 
+          {/* Folder kebab menu — only on folder rows. Hidden until hover
+           *  (or when its menu is open) so the count column stays aligned
+           *  in the resting state. */}
+          {item.id.startsWith('folder-') &&
+            (() => {
+              const folderId = item.id.slice('folder-'.length)
+              const isMenuOpen = openMenuId === item.id
+              return (
+                <div className="relative flex-shrink-0">
+                  <button
+                    onClick={(e) => {
+                      e.stopPropagation()
+                      setOpenMenuId(isMenuOpen ? null : item.id)
+                    }}
+                    className={clsx(
+                      'rounded p-0.5 text-text-muted hover:bg-surface-offset hover:text-text',
+                      isMenuOpen ? 'visible' : 'invisible group-hover:visible'
+                    )}
+                    title="More actions"
+                    aria-label="More folder actions"
+                    aria-haspopup="menu"
+                    aria-expanded={isMenuOpen}
+                  >
+                    <MoreHorizontal className="h-3.5 w-3.5" />
+                  </button>
+
+                  {isMenuOpen && (
+                    <div
+                      ref={menuRef}
+                      role="menu"
+                      onClick={(e) => e.stopPropagation()}
+                      className="absolute right-0 top-full z-30 mt-1 min-w-[180px] overflow-hidden rounded-lg border border-border bg-surface py-1 text-sm shadow-xl"
+                    >
+                      <FolderMenuItem
+                        icon={<FolderPlus className="h-3.5 w-3.5" />}
+                        label="New sub-folder"
+                        onClick={() => {
+                          setOpenMenuId(null)
+                          setCreatingUnder(folderId)
+                          setCreateDraft('')
+                          // Make sure the parent is expanded so the new
+                          // input is visible.
+                          if (!expandedItems.has(item.id)) {
+                            toggleExpanded(item.id)
+                          }
+                        }}
+                      />
+                      <FolderMenuItem
+                        icon={<Pencil className="h-3.5 w-3.5" />}
+                        label="Rename"
+                        onClick={() => {
+                          setOpenMenuId(null)
+                          setRenamingId(item.id)
+                          setRenameDraft(item.label)
+                        }}
+                      />
+                      <div className="my-1 h-px bg-border" />
+                      <FolderMenuItem
+                        icon={<Trash2 className="h-3.5 w-3.5" />}
+                        label="Delete folder…"
+                        destructive
+                        onClick={() => {
+                          setOpenMenuId(null)
+                          setDeletingFolder({
+                            id: folderId,
+                            name: item.label,
+                            photoCount: item.count,
+                          })
+                        }}
+                      />
+                    </div>
+                  )}
+                </div>
+              )
+            })()}
+
         </div>
 
+        {/* Inline "create new sub-folder" input. Renders just below the
+         *  parent row when its create state is active. */}
+        {item.id.startsWith('folder-') &&
+          creatingUnder === item.id.slice('folder-'.length) && (
+            <div
+              className="flex items-center gap-1 px-2 py-1"
+              style={{ paddingLeft: `${8 + (depth + 1) * 16 + 4}px` }}
+            >
+              <FolderPlus className="h-3 w-3 flex-shrink-0 text-text-muted" />
+              <input
+                autoFocus
+                type="text"
+                value={createDraft}
+                placeholder="New folder name"
+                onChange={(e) => setCreateDraft(e.target.value)}
+                onClick={(e) => e.stopPropagation()}
+                onKeyDown={(e) => {
+                  if (e.key === 'Enter') {
+                    const name = createDraft.trim()
+                    if (name) {
+                      createFolderMutation.mutate({
+                        parentId: item.id.slice('folder-'.length),
+                        name,
+                      })
+                    }
+                  } else if (e.key === 'Escape') {
+                    setCreatingUnder(null)
+                    setCreateDraft('')
+                  }
+                }}
+                onBlur={() => {
+                  // Don't auto-commit on blur — empty/escaped renames
+                  // close the input but don't fire the request.
+                  if (!createFolderMutation.isPending) {
+                    setCreatingUnder(null)
+                    setCreateDraft('')
+                  }
+                }}
+                className="flex-1 rounded border border-border bg-bg px-1 py-0 text-[13px] text-text focus:border-primary focus:outline-none"
+              />
+            </div>
+          )}
+
         {/* Render Children */}
         {hasChildren && isExpanded && (
           <div>
@@ -488,6 +689,46 @@ export function LeftSidebar() {
           </button>
         </div>
       )}
+
+      <DeleteFolderDialog
+        isOpen={!!deletingFolder}
+        folderName={deletingFolder?.name ?? ''}
+        photoCount={deletingFolder?.photoCount}
+        onClose={() => setDeletingFolder(null)}
+        onConfirm={(mode) => {
+          if (deletingFolder) {
+            deleteFolderMutation.mutate({ id: deletingFolder.id, mode })
+          }
+        }}
+      />
     </div>
   )
+}
+
+function FolderMenuItem({
+  icon,
+  label,
+  onClick,
+  destructive = false,
+}: {
+  icon: React.ReactNode
+  label: string
+  onClick: () => void
+  destructive?: boolean
+}) {
+  return (
+    <button
+      role="menuitem"
+      onClick={onClick}
+      className={clsx(
+        'flex w-full items-center gap-2 px-3 py-1.5 text-left text-xs transition-colors',
+        destructive
+          ? 'text-reject hover:bg-reject/10'
+          : 'text-text hover:bg-surface-2'
+      )}
+    >
+      <span className="text-text-muted">{icon}</span>
+      {label}
+    </button>
+  )
 }