Make the site preview toolbar a window drag region

[shaunandrews]

Related issues

• Related to Agentic UI: Foundation #3646

How AI was used in this PR

Claude Code identified the remaining toolbar delta between #3646 and trunk, and wrote the CSS change following the existing drag-region pattern in the session view header.

Proposed Changes

• The site preview toolbar now acts as a window drag region, so you can grab it to move the Studio window — matching how the chat header behaves and how browser toolbars typically work. This ports the last remaining browser-toolbar piece of Agentic UI: Foundation #3646 to trunk.
• The toolbar controls (back/forward/refresh, annotate, submit, open in browser) and the page title opt out of the drag region, so clicking them and hovering the title for its URL tooltip keep working as before.

Testing Instructions

• Enable the Agentic UI via the Studio → Beta Features menu and open a chat with the site preview panel visible.
• Press and drag on an empty area of the preview toolbar (e.g. beside the page title): the window should move.
• Verify the back/forward/refresh buttons, the annotate and submit buttons, and the open-in-browser button still respond to clicks.
• Hover the page title and confirm the URL tooltip still appears.

Pre-merge Checklist

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

🤖 Generated with Claude Code

[wpmobilebot]

📊 Performance Test Results

Comparing 92f2253 vs trunk

app-size

Metric	trunk	92f2253	Diff	Change
App Size (Mac)	1328.22 MB	1328.22 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	92f2253	Diff	Change
load	1699 ms	1662 ms	37 ms	⚪ 0.0%

site-startup

Metric	trunk	92f2253	Diff	Change
siteCreation	8068 ms	8042 ms	26 ms	⚪ 0.0%
siteStartup	3907 ms	3905 ms	2 ms	⚪ 0.0%

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Results are median values from multiple test runs.

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