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/lib/stores/theme.ts

@@ -0,0 +1,29 @@
+import { writable } from 'svelte/store';
+import { browser } from '$app/environment';
+
+type Theme = 'dark' | 'light';
+
+function loadTheme(): Theme {
+	if (!browser) return 'dark';
+	return (localStorage.getItem('theme') as Theme) ?? 'dark';
+}
+
+function applyTheme(theme: Theme) {
+	if (!browser) return;
+	if (theme === 'light') {
+		document.documentElement.setAttribute('data-theme', 'light');
+	} else {
+		document.documentElement.removeAttribute('data-theme');
+	}
+}
+
+export const themeStore = writable<Theme>(loadTheme());
+
+themeStore.subscribe((theme) => {
+	applyTheme(theme);
+	if (browser) localStorage.setItem('theme', theme);
+});
+
+export function toggleTheme() {
+	themeStore.update((t) => (t === 'dark' ? 'light' : 'dark'));
+}