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/components/PhotoGrid.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 	import Icon from './Icon.svelte';
+	import { density } from '$lib/stores/density.svelte';
 	import type { Photo } from '$lib/types';
 
 	interface Props {
@@ -13,7 +14,10 @@
 {#if photos.length === 0}
 	<p class="text-text-muted p-8 text-center">Пока нет фотографий.</p>
 {:else}
-	<ul class="grid grid-cols-2 gap-2 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-5">
+	<ul
+		class="grid gap-2"
+		style="grid-template-columns: repeat({density.cols}, minmax(0, 1fr));"
+	>
 		{#each photos as photo, i (photo.id)}
 			<li>
 				<button