Agentic UI: Add responsive preview modes and the More options menu

[bcotrim]

Related issues

• Fixes STU-2089

How AI was used in this PR

Implemented with Claude Code, ported from the Design Preview exploration (#3975). I reviewed the code and verified manually in the app.

Proposed Changes

• New More options (⋮) menu in the preview toolbar with a Responsive mode group: Fit pane (default), Mobile · 390×844, Tablet · 768×1024, Desktop · 1440×900, and Desktop + Mobile. Mobile and the split view add a Mobile orientation group (Portrait / Landscape).
• Presets render as fixed-size device frames on a dot-grid letterbox, scaled to fit the pane — browser responsive-design-mode style. Mobile uses real device emulation (CDP), so responsive themes behave like on-device.
• Desktop + Mobile shows both frames side by side on the same path, staying in sync on navigation.
• The menu is modal, so clicking anywhere outside — including the preview page — closes it.

Fit	Mobile	Tablet	Desktop	Desktop + Mobile

Testing Instructions

Full app restart (new main-process IPC handler), agentic UI, a running site.

1. Preview → ⋮ → try each mode; resize the panel — frames re-fit live.
2. Mobile: orientation group appears, Landscape flips to 844×390, and a responsive theme shows its phone layout.
3. Desktop + Mobile: same page in both panes; navigating one follows in the other.
4. With the menu open, click the preview page → it closes. ⋮ is hidden while the site is stopped.
5. Check light + dark.

Pre-merge Checklist

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

🤖 Generated with Claude Code

[wpmobilebot]

📊 Performance Test Results

Comparing 68a2066 vs trunk

app-size

Metric	trunk	68a2066	Diff	Change
App Size (Mac)	1375.36 MB	1375.50 MB	+0.14 MB	⚪ 0.0%

site-editor

Metric	trunk	68a2066	Diff	Change
load	768 ms	745 ms	23 ms	⚪ 0.0%

site-startup

Metric	trunk	68a2066	Diff	Change
siteCreation	6479 ms	6514 ms	+35 ms	⚪ 0.0%
siteStartup	2374 ms	2381 ms	+7 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)