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/stores/streamStore.svelte.ts

@@ -3,4 +3,5 @@ export const streamStore = $state({
   CertHash: [0],
   Width: 0,
   Height: 0,
+  StreamToken: '',
 });