Fix Stop Transcription button not updating after click

After calling POST /api/stop, the button stayed on "Stop Transcription"
because the state update depended on the WebSocket broadcast which can
be delayed or missed (event loop threading issue).

Fix: poll GET /api/status immediately after start/stop API calls to
update the UI state directly, rather than waiting for the WebSocket.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/stores/backend.svelte.ts

@@ -302,6 +302,7 @@ export const backendStore = {
   setPort,
   connect: connectWebSocket,
   disconnect,
+  pollStatus,
   apiUrl,
   apiFetch,
   apiGet,