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/services/api.ts

@@ -42,12 +42,40 @@ export const sourceFolders = {
     return response.data
   },
 
-  /** Rename the display label only — the on-disk path is controlled by
-   *  the docker mount and cannot be changed from the UI. */
+  /** Rename a folder. SourceRoot ids only update the display label;
+   *  Folder ids actually move the directory on disk and update every
+   *  descendant photo's filepath. */
   rename: async (folderId: string, name: string) => {
     const response = await api.patch(`/folders/${folderId}`, { name })
     return response.data
   },
+
+  /** Create a new sub-folder under an existing Folder. parent_id MUST
+   *  be a Folder row id (not a SourceRoot id). */
+  create: async (parentId: string, name: string) => {
+    const response = await api.post('/folders', {
+      name,
+      parent_id: parentId,
+    })
+    return response.data as { id: string; name: string; path: string; parent_id: string }
+  },
+
+  /** Delete a folder. mode=discard moves all photos under it to the
+   *  discard pile (recoverable) and leaves the folder + on-disk dir
+   *  alone. mode=permanent unlinks files, removes folder rows, and
+   *  rmtrees the directory — irreversible. */
+  delete: async (folderId: string, mode: 'discard' | 'permanent') => {
+    const response = await api.delete(`/folders/${folderId}`, {
+      params: { mode },
+    })
+    return response.data as {
+      status: string
+      mode: string
+      discarded?: number
+      deleted_photos?: number
+      file_errors?: number
+    }
+  },
 }
 
 // Photos API