feat(auth): add user authentication with roles, HMAC-signed cookies, and login/register UI

- Add `users` PocketBase collection (username, password_hash, role, created)
- Implement HMAC-SHA256 signed cookie auth in hooks.server.ts; token payload is userId:username:role
- Add User type, getUserByUsername, createUser (scrypt), loginUser (timing-safe) to pocketbase.ts
- Add login/register page with tabbed form UI and server actions
- Add logout route that clears the auth cookie
- Add layout.server.ts auth guard: redirect unauthenticated users to /login
- Extend App.Locals and App.PageData with role field
- Add AUTH_SECRET, POCKETBASE_ADMIN_EMAIL/PASSWORD to .env.example
- Install @types/node for Node crypto/scrypt types

ui/src/routes/logout/+page.server.ts

@@ -0,0 +1,11 @@
+import { redirect } from '@sveltejs/kit';
+import type { Actions } from './$types';
+
+const AUTH_COOKIE = 'libnovel_auth';
+
+export const actions: Actions = {
+	default: async ({ cookies }) => {
+		cookies.delete(AUTH_COOKIE, { path: '/' });
+		redirect(302, '/login');
+	}
+};