feat(xai): track OAuth quotas in dashboard

[fenix007]

Summary

• enable Quota Tracker for xAI OAuth connections using the Grok billing and subscription endpoints
• reuse the existing Grok CLI quota parser while keeping provider-specific error messages
• preserve remaining percentages for xAI quota bars and show the active plan when no numeric allotment is exposed

Why this works

The xAI OAuth grant already includes the grok-cli:access scope. A read-only check with a real xAI OAuth connection returned HTTP 200 from both /v1/billing?format=credits and /v1/user?include=subscription. No token or account identifiers are included in this PR.

API-key xAI connections remain excluded from Quota Tracker; only OAuth connections are eligible.

Validation

• npx vitest run unit/grok-cli-usage.test.js unit/usage-dispatch.test.js: 19 passed
• full non-live suite: 1303 passed / 45 existing failures; clean origin/master: 1298 passed / the same 45 failures
• npm run build
• targeted ESLint: no errors
• provider OAuth URL baseline: unchanged

Behavior

Accounts with on-demand or prepaid credit data render quota bars. Paid subscription accounts whose billing response does not expose a numeric included quota render the detected plan plus the existing explanatory message instead of inventing a limit.