feat: close tabs with middle mouse click

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):
https://github.com/open-webui/computer/compare/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.

Start with GroupTabBar.svelte and inspect the existing close handler and pointerdown behavior for workspace and Home tab groups. Verify the middle-button interaction, permanent-tab exclusions, and matching close behavior with the existing × control, then test the tab bar in the browser.