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/CoverPicker.svelte

@@ -0,0 +1,112 @@
+<script lang="ts">
+	import { api } from '$lib/api';
+	import Icon from './Icon.svelte';
+	import { albums } from '$lib/stores/albums.svelte';
+	import type { Album, Photo } from '$lib/types';
+
+	interface Props {
+		albumId: string;
+		onClose: () => void;
+		onSelected?: (album: Album) => void;
+	}
+
+	let { albumId, onClose, onSelected }: Props = $props();
+
+	let photos = $state<Photo[]>([]);
+	let loading = $state(true);
+	let error = $state('');
+	let busy = $state(false);
+
+	$effect(() => {
+		const id = albumId;
+		loading = true;
+		error = '';
+		api<{ photos: Photo[] }>(`/albums/${id}/photos?limit=100`)
+			.then((data) => (photos = data.photos))
+			.catch((e) => (error = (e as Error).message))
+			.finally(() => (loading = false));
+	});
+
+	async function choose(photo: Photo) {
+		if (busy) return;
+		busy = true;
+		error = '';
+		try {
+			const album = await albums.setCover(albumId, photo.id);
+			onSelected?.(album);
+			onClose();
+		} catch (e) {
+			error = (e as Error).message;
+		} finally {
+			busy = false;
+		}
+	}
+
+	function onKey(ev: KeyboardEvent) {
+		if (ev.key === 'Escape') onClose();
+	}
+</script>
+
+<svelte:window onkeydown={onKey} />
+
+<div
+	class="fixed inset-0 z-50 flex items-center justify-center bg-black/50 p-4"
+	role="dialog"
+	aria-modal="true"
+	aria-label="Выбор обложки"
+	tabindex="-1"
+	onclick={(e) => {
+		if (e.target === e.currentTarget) onClose();
+	}}
+	onkeydown={onKey}
+>
+	<div class="border-border bg-bg-elevated flex max-h-[85vh] w-full max-w-2xl flex-col rounded-xl border">
+		<div class="border-border flex items-center justify-between border-b px-4 py-3">
+			<h2 class="font-semibold">Выбрать обложку</h2>
+			<button
+				type="button"
+				class="text-text-muted hover:text-text inline-flex items-center justify-center rounded-md p-1"
+				aria-label="Закрыть"
+				title="Закрыть"
+				onclick={onClose}
+			>
+				<Icon name="close" />
+			</button>
+		</div>
+
+		<div class="min-h-0 flex-1 overflow-y-auto p-4">
+			{#if error}<p class="text-danger mb-3 text-sm">{error}</p>{/if}
+			{#if loading}
+				<p class="text-text-muted text-sm">Загрузка…</p>
+			{:else if photos.length === 0}
+				<p class="text-text-muted text-sm">В альбоме нет фотографий.</p>
+			{:else}
+				<ul class="grid grid-cols-3 gap-2 sm:grid-cols-4 md:grid-cols-5">
+					{#each photos as photo (photo.id)}
+						<li>
+							<button
+								type="button"
+								disabled={busy}
+								class="border-border bg-bg-muted block aspect-square w-full overflow-hidden rounded-lg border hover:border-primary disabled:opacity-50"
+								onclick={() => choose(photo)}
+							>
+								{#if photo.thumb_url}
+									<img
+										src={photo.thumb_url}
+										alt={photo.original_filename}
+										class="h-full w-full object-cover"
+										loading="lazy"
+									/>
+								{:else}
+									<span class="text-text-subtle flex h-full w-full items-center justify-center text-xs">
+										обработка…
+									</span>
+								{/if}
+							</button>
+						</li>
+					{/each}
+				</ul>
+			{/if}
+		</div>
+	</div>
+</div>