feat: add People view with person cards and click-to-filter

Replace the tag-grouping people section with a dedicated PeopleView:
- Grid of face cluster cards showing representative photo thumbnail,
  person name, and photo count
- Click a card → navigates to all-photos filtered by that person's tag
- Inline rename via pencil icon on hover
- Empty state when no faces have been clustered yet

Wired into App.tsx as a section-level route alongside Map and Duplicates.

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

frontend/src/components/layout/LeftSidebar.tsx

@@ -258,7 +258,7 @@ export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
         navigateToSection('tags', { groupBy: 'tag' })
         break
       case 'people':
-        navigateToSection('people', { groupBy: 'tag' })
+        navigateToSection('people', {})
         break
       case 'colors':
         navigateToSection('colors', { groupBy: 'color' })