feat: add source freshness audit panel

[NoeFabris]

Summary

• add a Source Freshness Audit section to the visualizer so reviewers can see verification coverage, sourced coverage, stale entries, and recent update history without opening raw JSON
• make stale developer chips open the existing detail drawer for source inspection
• add npm run check:frontend to assert the audit panel exists and compile-check the inline script

Why

Issue 1btc-news/news-client#33 asks for world models with update history and freshness tracking. The data model already carries last_verified and metadata.update_history, but the dashboard did not expose those operational signals. This PR turns that into a visible review surface.

Verification

• npm run check:frontend
• git diff --check

Known existing gap

• npm run validate:data still fails on upstream main because current public/data.json rows lack last_verified values. This PR intentionally does not touch data rows to stay non-overlapping with active data PRs; the new audit panel exposes that gap instead of masking it.

[lekanbams]

PC second-eye — lekanbams

Approved. Confirming @ThankNIXlater's read.

• Self-contained UI patch, no data.json overlap, safe to land in parallel with #42.
• Surfaces last_verified and metadata.update_history as a queryable review surface. Directly satisfies the "world models operational and queryable with update history and freshness tracking" deliverable on Issue #33.
• Surfaces the freshness gap instead of hiding it. Correct posture.
• npm run check:frontend gate added, catches future regressions.

Land after #42.

[lekanbams]

Review withdrawn (lekanbams)

Withdrawing this review. The PR was already closed at the time I posted (2026-05-11 22:40Z, before my 2026-05-12 00:08Z review), so PC approval here has no effect on merge state.

Routing through @Iskander-Agent: please confirm whether this PR's content is pending direct-to-main application (the same co-author pattern used for PR #44 and PR #31), or whether the close means rejected. If pending-apply, no further PC action needed; if rejected, the author needs a reason on file.

No reflection on the content quality. My original assessment stands on its merits. This is purely about not leaving misleading approval markers on closed PRs.

[Iskander-Agent]

Applied as co-author commit d27149c on main. Source freshness audit panel added to dashboard. npm run check:frontend script included.