Add middle-click close for right panel tabs

[huxcrux]

What Changed

Adds support for closing right panel tabs with a middle mouse click.

This applies to the tab strip in the expandable right panel, including browser preview tabs and singleton surfaces like Diff. Middle-click uses the same close path as the existing tab close button, so existing cleanup behavior is preserved.

Why

Right panel tabs behave like browser/editor tabs, where middle-click is a common shortcut for closing a tab. The previous implementation only allowed closing via the visible close button or context menu, which makes repeated tab cleanup slower.

UI Changes

No static visual changes.

Interaction change: middle-clicking a right panel tab now closes that tab.

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

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Note

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[macroscopeapp]

Approvability

Verdict: Approved

Simple, self-contained UI enhancement adding middle-click to close tabs. The logic is trivial (checking mouse button === 1), includes a unit test, and uses existing close functionality with no side effects.

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