Fix Google OAuth invalid_client setup + improve desktop guidance

[liuxiaopai-ai]

Fixes #353.

What changed

• Updated google-setup.md to recommend creating a Desktop app OAuth client (Rowboat uses a localhost loopback redirect) and added troubleshooting for Error 401: invalid_client.
• Improved the Google Client ID prompt to clarify what value to paste (Client ID ending in .apps.googleusercontent.com) and to not paste the client secret.
• When the OAuth flow times out for Google (e.g. user sees an error page in the browser and never reaches the localhost callback), Rowboat now emits a more actionable message pointing to the most common invalid_client causes + the setup guide.

Tests

• pnpm -C apps/x run deps
• pnpm -C apps/x/apps/main run build
• pnpm -C apps/x/apps/renderer run build

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