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>
