Fix widget source toggle refresh

[urbanlama]

Summary

• Keep the compact widget layout driven by source toggles even while usage data is temporarily unavailable.
• Preserve last-good usage snapshots on polling failures and render neutral placeholders instead of stale 0% or error text.
• Add widget-focused tests to cover source toggle and last-good fallback behavior.

Test Plan

• npm test
• npm run lint
• npm run --prefix widget lint
• npm run --prefix widget build