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/models/models.go

@@ -26,13 +26,17 @@ func (u *User) Public() PublicUser {
 }
 
 type Album struct {
-	ID           string    `json:"id"`
-	OwnerID      string    `json:"owner_id"`
-	Title        string    `json:"title"`
-	Description  string    `json:"description"`
-	CoverPhotoID *string   `json:"cover_photo_id,omitempty"`
-	CreatedAt    time.Time `json:"created_at"`
-	UpdatedAt    time.Time `json:"updated_at"`
+	ID            string    `json:"id"`
+	OwnerID       string    `json:"owner_id"`
+	Title         string    `json:"title"`
+	Description   string    `json:"description"`
+	CoverPhotoID  *string   `json:"cover_photo_id,omitempty"`
+	CoverThumbKey string    `json:"-"`
+	CoverThumbURL string    `json:"cover_thumb_url,omitempty"`
+	PhotosCount   int       `json:"photos_count"`
+	MembersCount  int       `json:"members_count"`
+	CreatedAt     time.Time `json:"created_at"`
+	UpdatedAt     time.Time `json:"updated_at"`
 }
 
 type AlbumMember struct {