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/app.d.ts

@@ -5,8 +5,11 @@ declare global {
 		// interface Error {}
 		interface Locals {
 			sessionId: string;
+			user: { id: string; username: string; role: string } | null;
+		}
+		interface PageData {
+			user?: { id: string; username: string; role: string } | null;
 		}
-		// interface PageData {}
 		// interface PageState {}
 		// interface Platform {}
 	}