feat(frontend): add root layout with auth guard and bottom navbar

Adds +layout.ts auth guard (redirects to /login when no token).
Adds bottom navbar with inline SVGs for Categories/Tags/Files/Pools/
Settings, active-route highlight (#343249), muted-to-bright color
transition. Adds theme store (dark/light, persisted to localStorage,
applies data-theme attribute). Hides navbar on /login route.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/+layout.ts

@@ -0,0 +1,16 @@
+import { get } from 'svelte/store';
+import { redirect } from '@sveltejs/kit';
+import { browser } from '$app/environment';
+import { authStore } from '$lib/stores/auth';
+
+export const ssr = false;
+
+export const load = ({ url }: { url: URL }) => {
+	if (!browser) return;
+	if (url.pathname === '/login') return;
+
+	const { accessToken } = get(authStore);
+	if (!accessToken) {
+		redirect(307, '/login');
+	}
+};