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/service.go

@@ -121,7 +121,7 @@ func (s *Service) processVariants(photoID, albumID string, data []byte) {
 	if err := s.store.SetVariants(ctx, photoID, thumbKey, previewKey); err != nil {
 		return
 	}
-	s.store.SetCover(ctx, albumID, photoID)
+	s.store.SetCoverIfEmpty(ctx, albumID, photoID)
 	if p, err := s.store.GetByID(ctx, photoID, ""); err == nil {
 		if err := s.attachURLs(ctx, p); err == nil {
 			s.hub.Publish(albumID, "photo.created", p)