feat: close tabs with middle mouse click

[ebifrier]

Feature description

Tabs in the group tab bar can currently only be closed with the × button or the context menu. Browsers and most editors also close a tab on middle mouse click, and muscle memory makes people try it here too. Right now a middle click does nothing (it just activates the tab).

Proposed solution

Close a non-permanent tab on middle click (auxclick with button === 1) in GroupTabBar.svelte, for both workspace and Home tab groups, reusing the existing close handler so the behaviour matches the × button.

Two small details worth handling at the same time:

• The pointerdown handler currently activates the tab for any mouse button. Restricting it to the primary button avoids briefly focusing a background tab before it is closed.
• Cancelling pointerdown for the middle button suppresses the browser's autoscroll cursor on Linux/Windows.

Permanent tabs (Home, Files) are excluded, same as they have no × button.

Additional context

Implemented on a fork; single-file frontend change (+18/-1):
main...ebifrier:computer:feat/middle-click-close-tab

Related: #254 (rename tabs via the tab-bar context menu) touches the same tab bar but is a separate feature.

[owui-terminator]

🔍 Related Issues Found

I found some existing issues that might be related. Please check if any of these are duplicates or contain helpful solutions:

1. 🟢 #254 feat: rename tabs via the tab-bar context menu
   This issue touches the same tab bar UI and the same non-permanent tab behavior, but for the context menu. It is related because both changes involve how tab-bar interactions expose tab actions, though it is a separate feature request.
   by 1-kabir

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💡 If your issue is a duplicate, please close it and add any additional details to the existing issue instead.

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