feat(usage): add xAI quota reporting

packages/ui/src/components/sections/usage/UsagePage.tsx\n  - What: Show selected configured-provider errors in Usage settings.\n  - Why: Preserve provider-specific failures when another provider succeeds.\n\npackages/ui/src/lib/quota/providers/index.ts\n  - What: Register xAI in shared quota metadata.\n  - Why: Expose xAI across shared usage surfaces.\n\npackages/ui/src/types/quota.ts\n  - What: Add xAI to the quota provider contract.\n  - Why: Allow typed xAI results across runtimes.\n\npackages/vscode/src/opencodeAuth.ts\n  - What: Preserve secure auth-file persistence for refreshed xAI credentials.\n  - Why: Reuse the VS Code auth boundary safely.\n\npackages/vscode/src/quotaProviders.ts\n  - What: Add xAI SuperGrok quota fetching, refresh, parsing, and failure isolation.\n  - Why: Provide VS Code usage parity.\n\npackages/web/server/lib/opencode/auth.js\n  - What: Enforce secure auth directory, auth file, and backup permissions.\n  - Why: Protect refreshed OAuth credentials.\n\npackages/web/server/lib/quota/DOCUMENTATION.md\n  - What: Document the xAI quota provider.\n  - Why: Keep provider ownership documentation current.\n\npackages/web/server/lib/quota/providers/index.js\n  - What: Register the xAI web quota adapter.\n  - Why: Expose xAI through the existing quota route.\n\npackages/web/server/lib/quota/providers/xai.js\n  - What: Add SuperGrok usage fetching, inherited OAuth refresh, strict parsing, and zero/error semantics.\n  - Why: Report xAI billing-period usage without new auth architecture or credentials.

packages/ui/src/lib/quota/providers/index.ts

@@ -24,4 +24,5 @@ export const QUOTA_PROVIDERS: QuotaProviderMeta[] = [
   { id: 'crof', name: 'CrofAI' },
   { id: 'deepseek', name: 'DeepSeek' },
   { id: 'neuralwatt', name: 'NeuralWatt' },
+  { id: 'xai', name: 'xAI' },
 ];