Add "Log in with browser" cloud device login

Connects the desktop app to Voicebox Cloud without the user ever handling an
API key. One button in Settings → General opens the system browser to
voicebox.sh, the user authorizes while signed in, and the credential lands
back in the app automatically.

Backend (FastAPI):
- /cloud/login/start opens the browser to the cloud authorize page with a
  state we mint; the existing loopback server catches the redirect at
  /cloud/callback and exchanges the one-time code (server-to-server, over TLS)
  for a voicebox_ API key, verifies it against the API, and stores it.
- /cloud/status and /cloud/disconnect back the settings UI.
- state round-trip guards against login-CSRF; the key never crosses a browser
  URL and is never exposed to the frontend (status returns a prefix only).
- CloudSettings singleton row; config gains VOICEBOX_CLOUD_URL /
  VOICEBOX_CLOUD_API_URL (default the prod hosts, overridable for dev).

Frontend (React):
- CloudSection in Settings → General: "Log in with browser", polls status,
  shows the connected device + a dashboard link. API keys are the advanced
  path only, surfaced in the web dashboard.

The key is stored in the local app DB for now; OS keychain is a marked
follow-up.

backend/routes/__init__.py

@@ -23,6 +23,7 @@ def register_routers(app: FastAPI) -> None:
     from .speak import router as speak_router
     from .mcp_bindings import router as mcp_bindings_router
     from .events import router as events_router
+    from .cloud import router as cloud_router
 
     app.include_router(health_router)
     app.include_router(profiles_router)
@@ -42,3 +43,4 @@ def register_routers(app: FastAPI) -> None:
     app.include_router(speak_router)
     app.include_router(mcp_bindings_router)
     app.include_router(events_router)
+    app.include_router(cloud_router)