feat: real recursive folder tree in the sidebar

The Folders section in the LeftSidebar previously rendered the flat
list of source roots — actual subdirectories were invisible. Now it
shows the full nested tree, click any node to filter, drop targets
work at every depth.

Backend
- New GET /folders/tree returning a list of root nodes (one per
  active SourceRoot). Each node is { id, name, path, photo_count,
  children: [...] } with children sorted alphabetically at every
  level. Walks Folder rows whose source_root_id matches and whose
  path is at or beneath the source root, then attaches them by
  parent path so partial scans don't break the tree.
- The source root's display label is overlaid on the root folder
  node so the top-level entry reads as "Library" instead of
  "/photos".
- list_photos folder_id filter now does descendant matching: when
  a Folder id is given, it includes the folder itself and every
  Folder whose path is a sep-prefixed descendant. Matches the
  Lightroom mental model: clicking "Library" or any parent folder
  shows everything beneath it. The existing source-root-id branch
  is unchanged.

Frontend
- New types/api.ts FolderTreeNode interface and sourceFolders.tree()
  helper.
- New hooks/useFolderTreeQuery.ts with a 30s staleTime and a
  findFolderInTree() walker for id-based name lookups.
- LeftSidebar drops the flat foldersData list and uses the tree
  query. folderNodeToTreeItem recursively maps backend nodes into
  the existing TreeItem shape; renderTreeItem already knew how to
  recurse into children, so the tree just works at any depth.
  Drop targets, drag-to-move, drag-to-copy, double-click rename,
  and active-state highlighting all carry over to nested folders.
- The renameMutation now also invalidates ['folders', 'tree'] so a
  source-root rename refreshes the tree label immediately.
- ActiveFilterChips switches to the tree query and uses the new
  findFolderInTree walker so the chip label resolves correctly for
  sub-folder filters too — not just top-level source roots.
- The "Scan all folders" button visibility now keys off the tree
  length instead of the flat folders length.

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

frontend/src/components/layout/LeftSidebar.tsx

@@ -12,12 +12,13 @@ import {
   Copy,
 } from 'lucide-react'
 import clsx from 'clsx'
-import { sourceFolders, library, photos as photosApi } from '../../services/api'
-import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
+import { sourceFolders, library, photos as photosApi, type FolderTreeNode } from '../../services/api'
+import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { toast } from '../ToastContainer'
 import { useFilterStore } from '../../store/filterStore'
 import { HeapsPanel } from '../heaps/HeapsPanel'
 import { PHOTO_DRAG_MIME } from '../timeline/PhotoThumbnail'
+import { useFolderTreeQuery } from '../../hooks/useFolderTreeQuery'
 
 interface TreeItem {
   id: string
@@ -148,17 +149,15 @@ export function LeftSidebar() {
     }
   }
   
-  // Fetch folders from API
-  const { data: foldersData } = useQuery({
-    queryKey: ['folders'],
-    queryFn: sourceFolders.list,
-  })
+  // Fetch the recursive folder tree (one root per active source root).
+  const { data: folderTree = [] } = useFolderTreeQuery()
 
   const renameMutation = useMutation({
     mutationFn: ({ id, name }: { id: string; name: string }) =>
       sourceFolders.rename(id, name),
     onSuccess: () => {
       queryClient.invalidateQueries({ queryKey: ['folders'] })
+      queryClient.invalidateQueries({ queryKey: ['folders', 'tree'] })
     },
     onError: (e: any) =>
       toast.error('Rename failed', e?.response?.data?.detail || e.message || 'Unknown error'),
@@ -198,6 +197,18 @@ export function LeftSidebar() {
     setExpandedItems(newExpanded)
   }
   
+  // Recursively map a backend FolderTreeNode into our generic TreeItem.
+  const folderNodeToTreeItem = (node: FolderTreeNode): TreeItem => ({
+    id: `folder-${node.id}`,
+    label: node.name,
+    icon: <Folder className="h-4 w-4" />,
+    count: node.photo_count,
+    type: 'folder',
+    children: node.children.length > 0
+      ? node.children.map(folderNodeToTreeItem)
+      : undefined,
+  })
+
   const libraryTree: TreeItem[] = [
     {
       id: 'library',
@@ -214,13 +225,7 @@ export function LeftSidebar() {
       id: 'folders',
       label: 'Folders',
       icon: <Folder className="h-4 w-4" />,
-      children: foldersData?.folders?.map((folder: any) => ({
-        id: `folder-${folder.id}`,
-        label: folder.name || folder.path.split('/').pop() || folder.path,
-        icon: <Folder className="h-4 w-4" />,
-        count: folder.photo_count,
-        type: 'folder',
-      })) || [],
+      children: folderTree.map(folderNodeToTreeItem),
     },
   ]
   
@@ -411,7 +416,7 @@ export function LeftSidebar() {
       </div>
       
       {/* Bottom Actions */}
-      {foldersData?.folders?.length > 0 && (
+      {folderTree.length > 0 && (
         <div className="border-t border-border p-3">
           <button
             onClick={handleScanAll}