Show Studio Code AI usage quota in the agentic Usage settings tab

[sejas]

Related issues

• Related to the agentic Settings Usage tab work on agentic-settings-usage-tab

How AI was used in this PR

AI assisted with porting the quota logic from the legacy settings panel and with automated review passes (simplification + code review); all changes were verified with typecheck, lint, and unit tests.

Proposed Changes

• Add AI Usage progress bar to Usage tab

Testing Instructions

1. npm start with the agentic UI enabled and log in to a WordPress.com account that has a Studio Code cost cap.
2. Open Settings → Usage: the AI credits section should show "X% of monthly limit used (resets on DATE)" with a progress bar reflecting the percentage.
3. Go offline (e.g. disable network): the section should show "You're currently offline".
4. Verify the legacy settings dialog (classic UI → Settings) still shows the same quota line — it now reads the schema from the shared module.

Before	After

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[bcotrim]

Thanks for the adjustment @sejas 🙇