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)

frontend/src/app.css

@@ -15,22 +15,26 @@
 	--primary-fg: 0 0% 100%;
 	--danger: 0 84% 60%;
 	--success: 142 71% 45%;
+	--chrome: 0 0% 98%;
+	--canvas: 0 0% 100%;
 	--shadow: 0 0% 0% / 0.08;
 }
 
-/* Тёмная */
+/* Тёмная (графит Lightroom) */
 [data-theme='dark'] {
-	--bg: 0 0% 7%;
-	--bg-elevated: 0 0% 11%;
-	--bg-muted: 0 0% 16%;
+	--bg: 0 0% 9%;
+	--bg-elevated: 0 0% 15%;
+	--bg-muted: 0 0% 18%;
 	--border: 0 0% 22%;
-	--text: 0 0% 96%;
+	--text: 0 0% 92%;
 	--text-muted: 0 0% 65%;
 	--text-subtle: 0 0% 45%;
 	--primary: 217 91% 60%;
 	--primary-fg: 0 0% 100%;
 	--danger: 0 84% 65%;
 	--success: 142 65% 50%;
+	--chrome: 0 0% 13%;
+	--canvas: 0 0% 9%;
 	--shadow: 0 0% 0% / 0.4;
 }
 
@@ -47,6 +51,8 @@
 	--primary-fg: 0 0% 100%;
 	--danger: 0 72% 55%;
 	--success: 142 55% 42%;
+	--chrome: 40 22% 92%;
+	--canvas: 40 20% 96%;
 	--shadow: 30 20% 20% / 0.12;
 }