feat: lightroom-style workspace UI with album covers

Backend:
- expose cover thumbnail URL on album responses (LEFT JOIN + presign)
- auto-assign cover only when empty; FK cover_photo_id -> photos ON DELETE SET NULL
- add photos_count / members_count to album responses; cover repo test

Frontend:
- split layouts into (app) shell (resizable rail + contextual panel) and (public)
- album rail + create-album dialog; album panel with cover picker / make-cover
- photo viewer route with filmstrip, replacing lightbox; share reuses it
- grid density slider, toolbar + drag-and-drop upload, dark graphite default

OpenSpec: add lightroom-workspace-ui change (proposal, design, specs, tasks)

backend/internal/photos/store.go

@@ -122,6 +122,8 @@ func (s *Store) Delete(ctx context.Context, id string) error {
 	return nil
 }
 
-func (s *Store) SetCover(ctx context.Context, albumID, photoID string) {
-	_, _ = s.pool.Exec(ctx, `UPDATE albums SET cover_photo_id = $2 WHERE id = $1`, albumID, photoID)
+func (s *Store) SetCoverIfEmpty(ctx context.Context, albumID, photoID string) {
+	_, _ = s.pool.Exec(ctx,
+		`UPDATE albums SET cover_photo_id = $2 WHERE id = $1 AND cover_photo_id IS NULL`,
+		albumID, photoID)
 }