feat: integrate native ChatGPT and Anthropic OAuth

[JollyRogerz]

Description

This PR introduces Native OAuth support for ChatGPT Plus and Claude Pro, allowing users to leverage their existing subscriptions directly in Rowboat without needing proxies.

Motivation

Providing an alternative to pay-as-you-go API keys significantly lowers the barrier to entry for users who already pay for AI subscriptions.

Testing Performed

• Built and type-checked frontend and backend.
• Manually verified the OAuth PKCE device and browser flows.
• Verified AI SDK requests successfully inherit the Bearer tokens.

Risk & Limitations

• These flows utilize reverse-engineered endpoints (auth.openai.com and claude.ai). If OpenAI or Anthropic change their private SSO implementations, these flows may break and require maintenance.

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[JollyRogerz]

Update: Antigravity (Google Cloud Code) Gateway Integration Added

This PR now includes a third OAuth provider beyond ChatGPT and Anthropic — Google's internal "Antigravity" Cloud Code gateway — which gives users free access to Gemini and Claude models using just a Google account (no API key required).

What's working

Core gateway client (apps/x/packages/core/src/models/antigravity-gateway.ts):

• Session initialization via loadCodeAssist / onboardUser (handles new user onboarding with long-running operation polling)
• Custom fetch wrapper that intercepts @ai-sdk/google requests and rewrites them into the Antigravity envelope format ({ model, project, user_prompt_id, request: <gemini body> })
• Buffered SSE unwrapper — properly handles SSE data that spans multiple TCP chunks (critical for large thoughtSignature base64 fields in thinking models)
• Request sanitization — removes empty parts: [] model messages and empty generationConfig: {} that cause 400 errors from the gateway
• Automatic OAuth token refresh using the gateway's embedded client credentials

Settings UI (settings-dialog.tsx):

• Antigravity shown as an OAuth-only provider (no API key option)
• Built-in model catalog: Gemini 3 Flash, Gemini 3 Pro (Low/High Thinking), Gemini 3.1 Pro (Low/High Thinking), Claude Sonnet 4.6, Claude Opus 4.6 (Thinking)
• "Sign in with Google" button using the existing browser OAuth flow

Bug fixes in this update:

• Status bar was being clipped by the fixed-position sidebar — moved StatusBar inside SidebarInset so it shifts correctly with the sidebar peer layout
• Dev startup: added unset ELECTRON_RUN_AS_NODE to the Electron start script (VS Code/Cursor terminals inherit this env var which forces Electron into plain Node.js mode)

Bugs fixed during development

Error	Root cause	Fix
loadCodeAssist failed (401)	OAuth tokens expire after ~1h, not auto-refreshed	Added refresh logic in createProvider before use
Invalid value at 'enabled_credit_types[0]'	Sent "G1" string instead of protobuf enum	Removed the field entirely (not required)
Request contains an invalid argument (400)	{ role: "model", parts: [] } is invalid for Gemini API	Sanitize request to remove empty messages
AI_JSONParseError on streaming	SSE data with long base64 thoughtSignature split across TCP chunks	Implemented proper line buffer across reader.read() calls

Tested

• Gemini 3 Flash ✅
• Gemini 3.1 Pro High (thinking) ✅ — responses render correctly including thinking tokens
• Claude Sonnet 4.6 via Antigravity — pending (model available in catalog)

Not included

• The test scripts (test-gateway.mjs, test-models.js/mjs) used during debugging are not committed