AI credits: Warn at 80% usage in the sidebar and above the Classic composer

[gcsecsey]

Related issues

• Related to STU-2338 — this is the usage-notice half. The purchase confirmation is in AI credits: Confirm a top-up only once the balance grows #4745.
• Stacked on Agentic UI: Warn in the composer at 90% AI credit usage and complete the exhausted state #4714

How AI was used in this PR

I used Opus 5 for implementation and tests.

Proposed Changes

This PR adds a dismissible warning at 80% to the sidebar, with a purchase CTA. It is also shown again at these points:

Event	Result
Dismiss at 80%, keep spending	stays hidden
A top-up drops usage, then it climbs back	warns again
App restart	warns again
Usage passes 90%	the composer strip from #4714 takes over

Classic has no sidebar cards, so its warning sits above the composer instead. It has no composer strip either, so that one slot covers the 90% step as well.

The copy is the approved wording from the exploration in #4446. Still I'd appreciate a final look @shaunandrews.

Testing Instructions

Use the balance setup steps in 236947-ghe-Automattic/wpcom.

1. Set usage to about 83%. Confirm the sidebar shows At 83% usage with Add AI credits, and the composer shows no strip.
2. Set usage to about 92%. Confirm the composer strip shows At 92% usage, and the sidebar shows nothing. This is the duplication the issue is about.
3. Set usage below 80%, and to 100%. Confirm neither surface warns, and that at 100% the composer locks out.
4. Dismiss at 83% and reload the window. Confirm it comes back.
5. Switch to Classic, open Studio Code, and repeat steps 1–4. The notice sits above the composer, and covers 90% too.

Check both color schemes, a narrow window, and an RTL locale such as Arabic.

Pre-merge Checklist

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