feat: discard view with restore and empty actions

Adds the destructive-action loop the discard concept needed:

- Click "Discarded" in the left sidebar → activates the discarded
  filter; the timeline reloads showing discarded photos.
- DiscardActionBar appears at the top of the timeline only when the
  discarded filter is active. Shows the count, a Restore button (when
  photos are selected), and an Empty discard pile button.
- Empty action goes through a ConfirmDialog (new tiny reusable modal,
  same overlay pattern as AddSourceFolderDialog).
- Restore goes through POST /api/v1/discard/restore.
- DELETE /api/v1/discard/empty now actually os.unlink()s the files
  from disk in addition to removing the DB rows. Per-file failures
  are logged and reported in the response so a single permission
  error doesn't abort the batch.

Other library nodes wired in passing:
- "All Photos"   → clearAll()
- "Rated"        → setRatingMin(1)
- "Flagged"      → setFlag('picked')
- "Discarded"    → setFlag('discarded')
- "By Date"      left unwired (needs a date-grouping UI)

Single-photo restore via the U keyboard shortcut already worked from
an earlier round, no change needed.

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

frontend/src/components/layout/LeftSidebar.tsx

@@ -18,6 +18,7 @@ import { AddSourceFolderDialog } from '../dialogs/AddSourceFolderDialog'
 import { sourceFolders, library } from '../../services/api'
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query'
 import { toast } from '../ToastContainer'
+import { useFilterStore } from '../../store/filterStore'
 
 interface TreeItem {
   id: string
@@ -33,8 +34,34 @@ export function LeftSidebar() {
   const [selectedItem, setSelectedItem] = useState<string | null>('all-photos')
   const [showAddFolderDialog, setShowAddFolderDialog] = useState(false)
   const [isScanning, setIsScanning] = useState(false)
-  
+
   const queryClient = useQueryClient()
+  const clearAllFilters = useFilterStore((s) => s.clearAll)
+  const setRatingMin = useFilterStore((s) => s.setRatingMin)
+  const setFlag = useFilterStore((s) => s.setFlag)
+
+  // Map a library tree id to a filter-store mutation. Each "virtual node" in
+  // the library tree is just a saved filter preset.
+  const applyLibraryNode = (id: string) => {
+    switch (id) {
+      case 'all-photos':
+        clearAllFilters()
+        break
+      case 'rated':
+        clearAllFilters()
+        setRatingMin(1)
+        break
+      case 'flagged':
+        clearAllFilters()
+        setFlag('picked')
+        break
+      case 'discarded':
+        clearAllFilters()
+        setFlag('discarded')
+        break
+      // 'by-date' is purely visual until we add a date-grouping UI
+    }
+  }
   
   // Fetch folders from API
   const { data: foldersData, refetch: refetchFolders } = useQuery({
@@ -152,6 +179,8 @@ export function LeftSidebar() {
             setSelectedItem(item.id)
             if (hasChildren) {
               toggleExpanded(item.id)
+            } else {
+              applyLibraryNode(item.id)
             }
           }}
         >