Add "Log in with browser" cloud device login (#812)

* 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.

* Address review feedback on cloud login

- time out status polling after 2 min so an abandoned browser flow
  doesn't leave the button stuck on "Waiting for browser…"
- handle non-JSON / non-object payloads from the exchange and account
  endpoints instead of 500ing after the state is consumed
- make singleton row creation race-safe (IntegrityError -> re-query)
- clear device_name on disconnect along with the rest of the metadata
- serve the dashboard URL from /cloud/status so the Manage link follows
  VOICEBOX_CLOUD_URL instead of hardcoding production
- keep a "Disconnecting…" label on the disconnect button while pending

* Remove orphaned react-qr-code entries from lockfile

bun.lock was out of date with package.json (react-qr-code was removed
without reinstalling), failing the frozen-lockfile install in CI.

bun.lock

@@ -57,7 +57,6 @@
         "react-dom": "^18.3.0",
         "react-hook-form": "^7.53.0",
         "react-i18next": "^17.0.4",
-        "react-qr-code": "^2.0.18",
         "react-sound-visualizer": "^1.4.0",
         "tailwind-merge": "^2.5.4",
         "wavesurfer.js": "^7.0.0",
@@ -1006,8 +1005,6 @@
 
     "punycode": ["[email protected]", "", {}, "sha512-vYt7UD1U9Wg6138shLtLOvdAu+8DsC/ilFtEVHcH+wydcSpNE20AfSOduf6MkRFahL5FY7X1oU7nKVZFtfq8Fg=="],
 
-    "qr.js": ["[email protected]", "", {}, "sha512-c4iYnWb+k2E+vYpRimHqSu575b1/wKl4XFeJGpFmrJQz5I88v9aY2czh7s0w36srfCM1sXgC/xpoJz5dJfq+OQ=="],
-
     "queue-microtask": ["[email protected]", "", {}, "sha512-NuaNSa6flKT5JaSYQzJok04JzTL1CA6aGhv5rfLW3PgqA+M2ChpZQnAC8h8i4ZFkBS8X5RqkDBHA7r4hej3K9A=="],
 
     "react": ["[email protected]", "", { "dependencies": { "loose-envify": "^1.1.0" } }, "sha512-wS+hAgJShR0KhEvPJArfuPVN1+Hz1t0Y6n5jLrGQbkb4urgPE/0Rve+1kMB1v/oWgHgm4WIcV+i7F2pTVj+2iQ=="],
@@ -1022,8 +1019,6 @@
 
     "react-loaders": ["[email protected]", "", { "dependencies": { "classnames": "^2.2.3" }, "peerDependencies": { "prop-types": ">=15.6.0", "react": ">=15" } }, "sha512-4igMNqs9Fb3d4Z+0UHIGQNJsw/37gX0nUO8QxupnEKRn1dtyYC1LGwk5GuaoDciMQCQc/MmPwb4Fn6ZfdoX1FQ=="],
 
-    "react-qr-code": ["[email protected]", "", { "dependencies": { "prop-types": "^15.8.1", "qr.js": "0.0.0" }, "peerDependencies": { "react": "*" } }, "sha512-v1Jqz7urLMhkO6jkgJuBYhnqvXagzceg3qJUWayuCK/c6LTIonpWbwxR1f1APGd4xrW/QcQEovNrAojbUz65Tg=="],
-
     "react-refresh": ["[email protected]", "", {}, "sha512-z6F7K9bV85EfseRCp2bzrpyQ0Gkw1uLoCel9XBVWPg/TjRj94SkJzUTGfOa4bs7iJvBWtQG0Wq7wnI0syw3EBQ=="],
 
     "react-remove-scroll": ["[email protected]", "", { "dependencies": { "react-remove-scroll-bar": "^2.3.7", "react-style-singleton": "^2.2.3", "tslib": "^2.1.0", "use-callback-ref": "^1.3.3", "use-sidecar": "^1.1.3" }, "peerDependencies": { "@types/react": "*", "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0 || ^19.0.0-rc" }, "optionalPeers": ["@types/react"] }, "sha512-Iqb9NjCCTt6Hf+vOdNIZGdTiH1QSqr27H/Ek9sv/a97gfueI/5h1s3yRi1nngzMUaOOToin5dI1dXKdXiF+u0Q=="],