feat: colors view and color label thumbnail ornament

Adds a Colors entry under Views that buckets photos in canonical
R-O-Y-G-B-P order (plus an Uncolored tail), mirroring the rated/tags
grouping pattern. Surfaces the color label on each thumbnail as a
small swatch chip preceding the rating badge in the BL corner so
labels are visible everywhere, not just inside the new view. Also
types color_label on the Photo interface — the backend already
returned it.

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

frontend/src/components/layout/LeftSidebar.tsx

@@ -10,6 +10,7 @@ import {
   HardDrive,
   Copy,
   Tag as TagIcon,
+  Palette,
   Layers2,
   MoreHorizontal,
   Pencil,
@@ -244,6 +245,9 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
       case 'tags':
         navigateToSection('tags', { groupBy: 'tag' })
         break
+      case 'colors':
+        navigateToSection('colors', { groupBy: 'color' })
+        break
       default:
         if (id.startsWith('folder-')) {
           const folderId = id.slice('folder-'.length)
@@ -344,6 +348,7 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
         { id: 'all-photos', label: 'All Photos', icon: <Image className="h-4 w-4" />, count: stats?.all_photos ?? 0 },
         { id: 'rated', label: 'Rated', icon: <Star className="h-4 w-4" />, count: stats?.rated ?? 0 },
         { id: 'tags', label: 'Tags', icon: <TagIcon className="h-4 w-4" />, count: tagsTotalCount },
+        { id: 'colors', label: 'Colors', icon: <Palette className="h-4 w-4" /> },
         { id: 'duplicates', label: 'Duplicates', icon: <Copy className="h-4 w-4" />, count: stats?.duplicates ?? 0 },
         { id: 'discarded', label: 'Discarded', icon: <Trash2 className="h-4 w-4" />, count: stats?.discarded ?? 0 },
       ],