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/repo/repo_test.go

@@ -12,6 +12,7 @@ import (
 	"shared-album/internal/auth"
 	"shared-album/internal/members"
 	"shared-album/internal/models"
+	"shared-album/internal/photos"
 	"shared-album/internal/sharelinks"
 )
 
@@ -123,6 +124,71 @@ func TestAlbumRepository(t *testing.T) {
 	}
 }
 
+func TestAlbumCoverRepository(t *testing.T) {
+	pool := testPool(t)
+	defer pool.Close()
+	ctx := context.Background()
+	store := albums.NewStore(pool)
+
+	owner := newUser(t, ctx, pool, "CoverOwner")
+	t.Cleanup(func() { _, _ = pool.Exec(ctx, `DELETE FROM users WHERE id = $1`, owner.ID) })
+
+	a := &models.Album{ID: auth.NewID(), OwnerID: owner.ID, Title: "Cover"}
+	if err := store.Create(ctx, a); err != nil {
+		t.Fatalf("create album: %v", err)
+	}
+
+	insertPhoto := func(id, thumbKey string) {
+		t.Helper()
+		if _, err := pool.Exec(ctx,
+			`INSERT INTO photos (id, album_id, uploader_id, storage_key, thumb_key)
+			 VALUES ($1, $2, $3, $4, $5)`,
+			id, a.ID, owner.ID, "albums/"+a.ID+"/original/"+id+".jpg", thumbKey); err != nil {
+			t.Fatalf("insert photo: %v", err)
+		}
+	}
+
+	photo1 := auth.NewID()
+	insertPhoto(photo1, "albums/"+a.ID+"/thumb/"+photo1+".webp")
+
+	got, err := store.GetByID(ctx, a.ID)
+	if err != nil || got.CoverThumbKey != "" {
+		t.Fatalf("expected empty cover key before set: %v %q", err, got.CoverThumbKey)
+	}
+
+	ps := photos.NewStore(pool)
+	ps.SetCoverIfEmpty(ctx, a.ID, photo1)
+	got, _ = store.GetByID(ctx, a.ID)
+	if got.CoverPhotoID == nil || *got.CoverPhotoID != photo1 {
+		t.Fatalf("cover not set: %+v", got.CoverPhotoID)
+	}
+	if want := "albums/" + a.ID + "/thumb/" + photo1 + ".webp"; got.CoverThumbKey != want {
+		t.Fatalf("cover thumb key = %q want %q", got.CoverThumbKey, want)
+	}
+
+	photo2 := auth.NewID()
+	insertPhoto(photo2, "thumb-2")
+	ps.SetCoverIfEmpty(ctx, a.ID, photo2)
+	got, _ = store.GetByID(ctx, a.ID)
+	if got.CoverPhotoID == nil || *got.CoverPhotoID != photo1 {
+		t.Fatalf("existing cover was overridden: %+v", got.CoverPhotoID)
+	}
+
+	if _, err := pool.Exec(ctx, `DELETE FROM photos WHERE id = $1`, photo1); err != nil {
+		t.Fatalf("delete cover photo: %v", err)
+	}
+	got, _ = store.GetByID(ctx, a.ID)
+	if got.CoverPhotoID != nil {
+		t.Fatalf("cover not cleared after delete: %+v", got.CoverPhotoID)
+	}
+
+	ps.SetCoverIfEmpty(ctx, a.ID, photo2)
+	list, err := store.ListForUser(ctx, owner.ID)
+	if err != nil || len(list) != 1 || list[0].CoverThumbKey != "thumb-2" {
+		t.Fatalf("list cover key: %v %+v", err, list)
+	}
+}
+
 func TestMemberRepository(t *testing.T) {
 	pool := testPool(t)
 	defer pool.Close()