frontend: attach auth credentials to all API requests

Add Authorization Bearer header to all fetch calls (apps, stream
start). Handle 401 responses by clearing token and redirecting to
login. Pass stream_token from the stream start response through to
the WebTransport URL as a query parameter for proxy authentication.

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

frontend/src/routes/Cover.svelte

@@ -25,6 +25,7 @@
 		streamStore.CertHash = streamData.CertHash;
 		streamStore.Width = streamData.Width;
 		streamStore.Height = streamData.Height;
+		streamStore.StreamToken = streamData.StreamToken;
 
 		console.log(`Stream data retrieved. Navigating to /stream.`);
 		await goto('/stream');