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/routes/(public)/invites/[token]/+page.svelte

@@ -0,0 +1,65 @@
+<script lang="ts">
+	import { page } from '$app/stores';
+	import { goto } from '$app/navigation';
+	import { api } from '$lib/api';
+	import Icon from '$lib/components/Icon.svelte';
+	import { auth } from '$lib/stores/auth.svelte';
+
+	const token = $derived($page.params.token);
+
+	let info = $state<{ album_id: string; role: string; expires_at: string } | null>(null);
+	let error = $state('');
+	let busy = $state(false);
+
+	$effect(() => {
+		api<{ invite: typeof info }>(`/invites/${token}`)
+			.then((data) => (info = data.invite))
+			.catch((e) => (error = (e as Error).message));
+	});
+
+	async function accept() {
+		busy = true;
+		error = '';
+		try {
+			const data = await api<{ album_id: string }>(`/invites/${token}/accept`, {
+				method: 'POST'
+			});
+			goto(`/albums/${data.album_id}`);
+		} catch (e) {
+			error = (e as Error).message;
+		} finally {
+			busy = false;
+		}
+	}
+</script>
+
+<div class="mx-auto max-w-md py-12 text-center">
+	<h1 class="mb-4 text-2xl font-bold">Приглашение в альбом</h1>
+	{#if error}
+		<p class="text-danger">{error}</p>
+	{:else if info}
+		<p class="text-text-muted mb-6">Вас приглашают с ролью <strong>{info.role}</strong>.</p>
+		{#if auth.isAuthenticated}
+			<button
+				type="button"
+				disabled={busy}
+				onclick={accept}
+				class="bg-primary text-primary-fg inline-flex items-center gap-2 rounded-lg px-5 py-2.5 font-medium"
+			>
+				<Icon name="check" class="size-5" />
+				{busy ? 'Принятие…' : 'Принять приглашение'}
+			</button>
+		{:else}
+			<p class="mb-4 text-sm">Войдите или зарегистрируйтесь, чтобы принять приглашение.</p>
+			<a
+				href="/login"
+				class="bg-primary text-primary-fg inline-flex items-center gap-2 rounded-lg px-5 py-2.5 font-medium"
+			>
+				<Icon name="login" class="size-5" />
+				Войти
+			</a>
+		{/if}
+	{:else}
+		<p class="text-text-muted">Загрузка…</p>
+	{/if}
+</div>