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/lib/stores/density.svelte.ts

@@ -0,0 +1,20 @@
+const KEY = 'grid-cols';
+const MIN = 2;
+const MAX = 8;
+
+class DensityStore {
+	cols = $state(5);
+
+	init() {
+		if (typeof window === 'undefined') return;
+		const v = parseInt(localStorage.getItem(KEY) ?? '', 10);
+		if (v >= MIN && v <= MAX) this.cols = v;
+	}
+
+	set(value: number) {
+		this.cols = Math.min(MAX, Math.max(MIN, Math.round(value)));
+		if (typeof window !== 'undefined') localStorage.setItem(KEY, String(this.cols));
+	}
+}
+
+export const density = new DensityStore();