Show the What's New announcements in the agentic UI

[bcotrim]

Related issues

• Related to STU-1988
• Stacked on Agentic UI: Add a first-run orientation guide #4331 — review that first; this PR targets its branch.

How AI was used in this PR

Claude wrote the code and I reviewed it. Worth a careful look at the shared lastSeenVersion wiring (both UIs now read and write the same marker) and at the constant-height grid in the guide modal.

Proposed Changes

The "What's New" announcements now appear in the agentic UI, reusing the orientation guide's modal from #4331 instead of a second paged component.

Both UIs are driven by the same switch and the same stored marker: flipping FORCE_SHOW_WHATS_NEW shows the announcements in whichever UI you're running, and dismissing them in one settles it for the other. Nothing about the classic renderer's behaviour changes.

A first-run user gets the orientation guide only — the announcements record themselves as seen without showing, so nobody meets two modals in a row. Classic already worked this way.

Two fixes to the shared modal, both visible in #4331's guide too: the dialog focused "Learn more" instead of the advance button, so Enter opened the docs rather than paging; and the modal resized as you advanced, which it no longer does — every page renders in one grid cell, so the height comes from the longest page and stays put across translations.

The five illustrations move to packages/common so both UIs render the same artwork rather than duplicating ~275 KB of SVG.

Testing Instructions

Agentic UI:

1. npm run cli:build:ui && node apps/cli/dist/cli/main.mjs ui --no-open, open http://localhost:8081
2. In devtools, localStorage.setItem('studio-onboarding-hints', JSON.stringify({tourCompletedVersion: 2})) and localStorage.removeItem('studio-whats-new-last-seen-version'), then reload — the guide opens after ~0.5s
3. Page through all five: artwork renders, the modal height never changes, "Learn more" opens the docs, Enter advances
4. Finish or skip, reload — it does not come back
5. Clear studio-onboarding-hints entirely and reload — you get the orientation guide only, and the announcements mark themselves seen
6. Repeat in light and dark

Desktop (needs npm start):

7. Help ▸ What's New reopens it in the agentic UI
8. Classic UI: the modal still shows its illustrations after the asset move

Pre-merge Checklist

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