Agentic UI: Add a preview zoom option

[shaunandrews]

Related issues

None. Follow-up to #4573, which stopped the app's zoom from leaking into the preview; this branch is stacked on it and will be retargeted to trunk once that merges.

How AI was used in this PR

Claude Code implemented the option, the emulation math, and the tests, following the direction from the review of #4573. Reviewed through the diff, lint, type check, unit tests, and manual testing in the macOS development app.

Proposed Changes

• Adds a Zoom group (50%–200%, default 100%) to the preview's More options menu while Fit pane is selected, the way the phone frame brings its orientation group.
• The page zooms like a browser: it reflows at the pane size over the zoom and renders back up to fill the pane.
• The device presets are already frames scaled to fit, so zoom doesn't apply to them; the group gives way while one is picked and the level comes back with Fit pane.
• Zoom is remembered per site for the session, alongside the responsive mode and orientation.
• The preview itself stays at 1:1 in Electron terms — everything goes through the same device-metrics emulation the presets already use — so the guest zoom pinning from Agentic UI: Fix zoom shortcuts with focused preview #4573 and this option don't fight.

Screenshots

The Zoom group, shown while Fit pane is selected:

Light	Dark

The same pane at 100% and at 150% — the page reflows narrower and renders larger, still filling the pane:

100%	150%

Testing Instructions

1. npm start, open a running site in the agentic UI.
2. More options → Zoom → 150%. The page should reflow narrower and render larger, still filling the pane. 100% returns it to normal.
3. Pick Desktop: the Zoom group disappears and the frame fits the pane as before.
4. Pick Fit pane again: 150% is still selected and applied.
5. Zoom the app UI (⌘+ / ⌘−): the preview zoom is unaffected.
6. Switch to another site and back: the zoom level is remembered.

Automated verification:

• npx eslint --fix on all modified files
• npm test -- apps/ui/src/components/site-preview/index.test.tsx apps/studio/src/tests/ipc-handlers.test.ts (69 tests passed, then 51 in the preview file after the Fit-pane scoping)
• npm run typecheck

Pre-merge Checklist

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