feat: Tags entry in the Library sidebar

Adds an expandable Tags node alongside All Photos / Rated /
Duplicates / Discarded. The children are populated dynamically from
useTagsQuery — one row per tag, showing the tag name and its photo
count badge. Click a tag row to filter the timeline to just that
tag (single-tag), with the active highlight following the filter
store.

Multi-tag filtering still lives in the FilterBar; the sidebar entry
is the quick "show me everything in this tag" affordance.

Implementation
- New 'tags' library tree node with children: allTags.map(...)
- 'tag-{id}' click handler in applyLibraryNode → clearAll() +
  setTagIds([id])
- isItemActive recognises a tag row as selected only when the
  filter store has exactly that single tag id, so combining it with
  multi-tag filter mode in the FilterBar doesn't leave a stale
  highlight.
- Tags section is collapsed by default like other library nodes; no
  effect when there are no tags yet (children list is empty).

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

frontend/src/components/layout/LeftSidebar.tsx

@@ -10,6 +10,7 @@ import {
   HardDrive,
   RefreshCw,
   Copy,
+  Tag as TagIcon,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { sourceFolders, library, photos as photosApi, type FolderTreeNode } from '../../services/api'
@@ -19,6 +20,7 @@ import { useFilterStore } from '../../store/filterStore'
 import { HeapsPanel } from '../heaps/HeapsPanel'
 import { PHOTO_DRAG_MIME } from '../timeline/PhotoThumbnail'
 import { useFolderTreeQuery } from '../../hooks/useFolderTreeQuery'
+import { useTagsQuery } from '../../hooks/useTagsQuery'
 
 interface TreeItem {
   id: string
@@ -44,8 +46,11 @@ export function LeftSidebar() {
   const setFlag = useFilterStore((s) => s.setFlag)
   const setFolderId = useFilterStore((s) => s.setFolderId)
   const setDuplicates = useFilterStore((s) => s.setDuplicates)
+  const setTagIds = useFilterStore((s) => s.setTagIds)
   const filterFolderId = useFilterStore((s) => s.folderId)
   const filterDuplicates = useFilterStore((s) => s.duplicates)
+  const filterTagIds = useFilterStore((s) => s.tagIds)
+  const { data: allTags = [] } = useTagsQuery()
   const [dropTargetId, setDropTargetId] = useState<string | null>(null)
 
   // Bulk discard mutation for the drag-onto-Discarded interaction.
@@ -138,6 +143,14 @@ export function LeftSidebar() {
         setDuplicates(true)
         break
       default:
+        if (id.startsWith('tag-')) {
+          // Tag rows: filter to that single tag. Multi-tag filtering is
+          // available via the FilterBar.
+          const tagId = id.slice('tag-'.length)
+          clearAllFilters()
+          setTagIds([tagId])
+          return
+        }
         if (id.startsWith('folder-')) {
           // Folder rows: filter to that folder, clear other filters that
           // would compete (heap, discarded, etc.) so the user sees what they
@@ -219,6 +232,17 @@ export function LeftSidebar() {
         { id: 'rated', label: 'Rated', icon: <Star className="h-4 w-4" />, count: 0 },
         { id: 'duplicates', label: 'Duplicates', icon: <Copy className="h-4 w-4" />, count: 0 },
         { id: 'discarded', label: 'Discarded', icon: <Trash2 className="h-4 w-4" />, count: 0 },
+        {
+          id: 'tags',
+          label: 'Tags',
+          icon: <TagIcon className="h-4 w-4" />,
+          children: allTags.map((tag) => ({
+            id: `tag-${tag.id}`,
+            label: tag.name,
+            icon: <TagIcon className="h-4 w-4" />,
+            count: tag.photo_count,
+          })),
+        },
       ],
     },
     {
@@ -237,6 +261,10 @@ export function LeftSidebar() {
     if (id.startsWith('folder-')) {
       return filterFolderId === id.slice('folder-'.length)
     }
+    if (id.startsWith('tag-')) {
+      const tagId = id.slice('tag-'.length)
+      return filterTagIds.length === 1 && filterTagIds[0] === tagId
+    }
     if (id === 'all-photos') {
       return filterFolderId === null && selectedItem === 'all-photos'
     }