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/tailwind.config.ts

@@ -11,6 +11,8 @@ export default {
 					muted: 'hsl(var(--bg-muted) / <alpha-value>)'
 				},
 				border: 'hsl(var(--border) / <alpha-value>)',
+				chrome: 'hsl(var(--chrome) / <alpha-value>)',
+				canvas: 'hsl(var(--canvas) / <alpha-value>)',
 				text: {
 					DEFAULT: 'hsl(var(--text) / <alpha-value>)',
 					muted: 'hsl(var(--text-muted) / <alpha-value>)',