feat(xai): add xAI Grok provider with OAuth + API key auth + image

Adapted from PR #1286 (mugnimaestra/feat/xai-grok-provider) to match
existing app architecture. Includes:

- OAuth 2.0 with PKCE on loopback port 56121 (Grok Build)
- API key auth path (console.x.ai)
- Token refresh wiring (open-sse + sse tokenRefresh)
- Dashboard OAuth modal with fixed-port flow + manual code fallback
- Provider registry entries (OAuth + API key)
- xAI image generation via OpenAI-compatible adapter
  (grok-2-image-1212 model, no size/quality/style params)

Excludes (intentionally, to match app patterns):
- Custom xAI Responses executor (DefaultExecutor handles /chat/completions)
- xAI-specific translators (app uses OpenAI as intermediate format)
- Image edits (not supported by current imageGenerationCore)
- Video endpoints (app has no video subsystem yet)
- CLI xai-login command

Refs decolua#1286

Co-authored-by: Cursor <cursoragent@cursor.com>

open-sse/handlers/imageProviders/index.js

@@ -17,6 +17,7 @@ const ADAPTERS = {
   minimax: createOpenAIAdapter("minimax"),
   openrouter: createOpenAIAdapter("openrouter"),
   recraft: createOpenAIAdapter("recraft"),
+  xai: createOpenAIAdapter("xai"),
   gemini,
   codex,
   sdwebui,