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>

src/lib/providerNormalization.js

@@ -1,5 +1,14 @@
 import { AI_PROVIDERS } from "../shared/constants/providers.js";
 
+/**
+ * Detect xAI Grok models by id pattern (grok-*, Grok_*, etc).
+ * @param {string} modelId
+ * @returns {boolean}
+ */
+export function isXaiModel(modelId) {
+  return typeof modelId === "string" && /^grok[-_]/i.test(modelId.trim());
+}
+
 export function normalizeProviderId(provider) {
   if (typeof provider !== "string") return provider;