feat: refactor grouping views into card-grid browse pattern

Replace Timeline-based grouped views (tags, colors, rated) with
dedicated card-grid components that drill into Timeline detail views
on click/Enter. Adds shared useCardGridNav hook for arrow-key
navigation across all four card grids (tags, colors, rated, people).

- TagsView, ColorsView, RatedView: card grid → inline Timeline detail
- PeopleView: migrated to same pattern (Timeline replaces custom grid)
- Tags endpoint: fall back to first associated photo for representative
- Filter store: add ratingMax for exact rating filtering in RatedView
- Timeline: remove tag/rating/color grouping; skip date headers when
  groupBy != 'date' so detail views render flat grids
- SettingsDialog: bump z-index above Leaflet map layers

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', {})
+        navigateToSection('people', { groupBy: 'tag' })
         break
       case 'colors':
         navigateToSection('colors', { groupBy: 'color' })