feat: auto-focus first photo on timeline mount

Arrow-key navigation required a click to establish an active photo
first. Now the grid auto-selects photos[0] on initial mount when no
active photo is set, so the user can land on the app and immediately
walk the grid with arrows. Guarded on viewMode === 'grid' and
!activePhotoId so we never clobber an existing selection or fight
with PreviewView.

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

frontend/src/components/timeline/Timeline.tsx

@@ -220,6 +220,17 @@ export function Timeline() {
   // they share one cache entry, regardless of filter state.
   const { data: photos = [], isLoading } = usePhotosQuery()
 
+  // Auto-focus the first photo on initial grid load so arrow-key nav
+  // works immediately without a pre-click. Only fires when there's no
+  // current active photo — we never clobber the user's selection or
+  // the one they restored by navigating back from preview.
+  useEffect(() => {
+    if (viewMode !== 'grid') return
+    if (activePhotoId) return
+    if (photos.length === 0) return
+    selectPhoto(photos[0].id)
+  }, [viewMode, activePhotoId, photos, selectPhoto])
+
   // Membership in the active heap (for the basket affordance). Subscribed
   // once at this level so we don't have hundreds of thumbnails each
   // subscribing to the same query.