feat(xai): track SuperGrok weekly limit + API usage quota

Fetch OAuth quota from cli-chat-proxy billing, GetGrokCreditsConfig weekly
window, and settings plan label so the dashboard matches grok.com usage.

tests/unit/usage-dispatch.test.js

@@ -15,7 +15,7 @@ const load = () => import("../../open-sse/services/usage.js");
 const SUPPORTED = [
   "github", "gemini-cli", "antigravity", "claude", "codex", "kiro",
   "qoder", "qwen", "iflow", "ollama", "glm", "glm-cn",
-  "minimax", "minimax-cn", "vercel-ai-gateway",
+  "minimax", "minimax-cn", "vercel-ai-gateway", "xai",
 ];
 
 describe("usage dispatch", () => {