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>

backend/app/routers/folders.py

@@ -44,6 +44,81 @@ async def get_folders(db: AsyncSession = Depends(get_db)):
     
     return {"folders": folders_list}
 
+@router.get("/tree")
+async def get_folder_tree(db: AsyncSession = Depends(get_db)):
+    """Recursive folder tree, one root per active SourceRoot. The tree
+    starts at the Folder row matching the SourceRoot.path (the scanner
+    creates one for every walked directory), with the SourceRoot's
+    display name overlaid so the top-level entry reads as "Library"
+    instead of "/photos".
+
+    Returns a list of root nodes; each node has:
+        { id, name, path, photo_count, children: [...] }
+
+    Sub-folders that physically belong to the same source root but
+    weren't created on disk (e.g. the / row the scanner sometimes
+    creates as a parent walk) are skipped via path-prefix filtering.
+    """
+    sr_result = await db.execute(
+        select(SourceRoot).where(SourceRoot.is_active == True)  # noqa: E712
+    )
+    source_roots = sr_result.scalars().all()
+
+    out = []
+    for sr in source_roots:
+        # Folders physically inside this source root, by path prefix.
+        prefix = os.path.normpath(sr.path).rstrip(os.sep)
+        f_result = await db.execute(
+            select(Folder).where(
+                Folder.source_root_id == sr.id,
+                # Either the folder IS the source root, or it sits beneath it.
+                (Folder.path == prefix) | (Folder.path.like(prefix + os.sep + '%'))
+            )
+        )
+        folders = f_result.scalars().all()
+        if not folders:
+            continue
+
+        # Build a path → node map so we can attach children regardless of
+        # parent_id consistency.
+        nodes = {
+            f.path: {
+                "id": f.id,
+                "name": f.name or os.path.basename(f.path),
+                "path": f.path,
+                "photo_count": f.photo_count or 0,
+                "children": [],
+            }
+            for f in folders
+        }
+
+        root_node = None
+        for f in folders:
+            node = nodes[f.path]
+            if f.path == prefix:
+                root_node = node
+                # Override the display name with the source root's label.
+                node["name"] = sr.name or node["name"]
+                continue
+            parent_path = os.path.normpath(os.path.dirname(f.path))
+            parent = nodes.get(parent_path)
+            if parent is not None:
+                parent["children"].append(node)
+            # If parent isn't in the set (orphan from a partial scan), drop
+            # the node — it can't be rendered consistently.
+
+        if root_node is not None:
+            # Sort children alphabetically at every level.
+            def sort_recursive(n):
+                n["children"].sort(key=lambda c: c["name"].lower())
+                for c in n["children"]:
+                    sort_recursive(c)
+            sort_recursive(root_node)
+            out.append(root_node)
+
+    return out
+
+
 @router.patch("/{folder_id}")
 async def rename_folder(
     folder_id: str,

backend/app/routers/photos.py

@@ -71,15 +71,17 @@ async def list_photos(
     if date_to:
         filters.append(Photo.taken_at <= date_to)
     
-    # Folder filter — the sidebar exposes "source roots" (top-level scan
-    # paths) under the same UI affordance as folders, so the same param has
-    # to accept either a folder id or a source root id. If the value matches
-    # a source root, expand to every folder under that root and use IN.
+    # Folder filter. The sidebar can pass either a SourceRoot id or a
+    # Folder id; both should include descendants so clicking a parent
+    # folder shows everything under it (Lightroom semantics).
     if folder_id:
         sr_check = await db.execute(
-            select(SourceRoot.id).where(SourceRoot.id == folder_id)
+            select(SourceRoot).where(SourceRoot.id == folder_id)
         )
-        if sr_check.scalar_one_or_none() is not None:
+        sr_row = sr_check.scalar_one_or_none()
+
+        if sr_row is not None:
+            # Source root → all folders under it (any depth).
             child_folders = await db.execute(
                 select(Folder.id).where(Folder.source_root_id == folder_id)
             )
@@ -87,11 +89,25 @@ async def list_photos(
             if child_ids:
                 filters.append(Photo.folder_id.in_(child_ids))
             else:
-                # Source root with no folder rows yet — match nothing rather
-                # than returning the entire library.
                 filters.append(Photo.id == '__no_match__')
         else:
-            filters.append(Photo.folder_id == folder_id)
+            # Folder id → that folder + every descendant by path prefix.
+            target_check = await db.execute(
+                select(Folder).where(Folder.id == folder_id)
+            )
+            target = target_check.scalar_one_or_none()
+            if target is None:
+                filters.append(Photo.id == '__no_match__')
+            else:
+                target_path = os.path.normpath(target.path).rstrip(os.sep)
+                desc_result = await db.execute(
+                    select(Folder.id).where(
+                        (Folder.path == target_path)
+                        | (Folder.path.like(target_path + os.sep + '%'))
+                    )
+                )
+                desc_ids = [row[0] for row in desc_result.all()]
+                filters.append(Photo.folder_id.in_(desc_ids))
     
     # Media type filter
     if media_type: