feat: group rated view by star rating

Mirrors the tags view: rated section now buckets photos 5★→1★ with
an "Unrated" tail group, instead of a flat ratingMin=1 stream.

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

frontend/src/components/layout/LeftSidebar.tsx

@@ -233,7 +233,7 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
         navigateToSection('all-photos', {})
         break
       case 'rated':
-        navigateToSection('rated', { ratingMin: 1 })
+        navigateToSection('rated', { ratingMin: 1, groupBy: 'rating' })
         break
       case 'discarded':
         navigateToSection('discarded', { flag: 'discarded' })