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/dialogs/SettingsDialog.tsx

@@ -195,7 +195,7 @@ export function SettingsDialog({ isOpen, onClose }: SettingsDialogProps) {
   if (!isOpen) return null
 
   return (
-    <div className="fixed inset-0 z-50">
+    <div className="fixed inset-0 z-[2000]">
       <div
         className="absolute inset-0 bg-black/60 backdrop-blur-sm"
         onClick={onClose}