frontend: add login page and auth guard

Add authentication flow to the frontend:
- authStore with token management (localStorage persistence)
- Login page with username/password form at /login
- Layout-level auth guard that redirects to /login when no valid
  session exists, validates token on load via GET /api/auth/me
- Top navigation bar showing username and admin link when applicable

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

gamestream-webtransport-proxy/src/proxy/handler.rs

@@ -85,31 +85,17 @@ impl crate::proxy::Proxy {
             description: "Could not start stream".to_string(),
         });
 
-        // Validate single-use stream token
+        // Validate single-use stream token via the shared helper so this
+        // handler and its unit tests exercise the same code path.
         let provided_token = req.query::<String>("token").unwrap_or_default();
-        {
-            let mut token_guard = self.stream_token.write().await;
-            match token_guard.take() {
-                Some(expected) if expected == provided_token => {
-                    // Token consumed successfully (single-use)
+        if let Err(msg) = super::validate_stream_token(&self, &provided_token).await {
+            error!("Stream token validation failed: {msg}");
+            return Err(AppError {
+                status_code: StatusCode::UNAUTHORIZED,
+                description: msg,
+            });
+        }
         info!("Stream token validated and consumed");
-                }
-                Some(_) => {
-                    error!("Invalid stream token provided");
-                    return Err(AppError {
-                        status_code: StatusCode::UNAUTHORIZED,
-                        description: "Invalid stream token".to_string(),
-                    });
-                }
-                None => {
-                    error!("Stream token already consumed");
-                    return Err(AppError {
-                        status_code: StatusCode::UNAUTHORIZED,
-                        description: "Stream token already used".to_string(),
-                    });
-                }
-            }
-        }
 
         info!("WebTransport connection initiated");
         let (wt_stream_send, wt_stream_recv, wt_datagram_send) = match setup_webtransport(req).await