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>

backend/app/routers/tags.py

@@ -48,12 +48,13 @@ async def list_tags(
         select(
             photo_tags.c.tag_id,
             func.count(photo_tags.c.photo_id).label("photo_count"),
+            func.min(photo_tags.c.photo_id).label("first_photo_id"),
         )
         .group_by(photo_tags.c.tag_id)
         .subquery()
     )
     stmt = (
-        select(Tag, count_subq.c.photo_count)
+        select(Tag, count_subq.c.photo_count, count_subq.c.first_photo_id)
         .outerjoin(count_subq, Tag.id == count_subq.c.tag_id)
     )
     if kind:
@@ -70,10 +71,10 @@ async def list_tags(
             "color": tag.color,
             "kind": tag.kind,
             "source": tag.source,
-            "representative_photo_id": tag.representative_photo_id,
+            "representative_photo_id": tag.representative_photo_id or first_photo_id,
             "photo_count": int(count or 0),
         }
-        for tag, count in rows
+        for tag, count, first_photo_id in rows
     ]