fix(web): hide browser when the right panel starts closing

[Neel2107]

What Changed

Pass visible={rightPanelOpen} to PreviewPanel so the docked browser hides as soon as the right panel starts closing, matching the terminal panel.

Why

With panel motion enabled, the panel stays mounted during its closing animation. The desktop webview previously stayed visible throughout that animation and lingered over the expanding chat. This uses the existing surface visibility handling, including parking inactive macOS guests offscreen.

Fixes #10383. Implements the immediate-hide fix suggested in Julius's triage.

Validation

• 15 focused browser surface, hosted webview, and visibility style tests passed.
• Web typecheck, targeted formatting, and desktop build passed. Targeted lint reported existing warnings.
• Tested in an isolated macOS desktop instance with panel motion set to 400 ms: close, reopen, and interaction with the reopened page. Reporter also tested and supplied the after recording.

UI Changes

Before: browser lingers over chat	After: browser hides during collapse

Before:

Screen.Recording.2026-09-06.at.8.33.44.PM.mov

After:

browser-panel-close-after.mov

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

Model: GPT-6-Astra. Harness: Codex.

[macroscopeapp]

Approvability

Verdict: Would Approve

Macroscope's review found this PR approvable — This narrowly scoped fix connects the preview browser’s visibility to the right-panel state, preventing the browser surface from remaining visible during panel closure. It changes only one existing UI path and introduces no broader capability or configuration impact.

Not approved because:

• Monthly spending limit reached (workspace setting). Approvability relies on correctness review in order to determine eligibility

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