Close sidebar tabs with middle click

[yusuf8834]

Middle-clicking a tab in the sidebar currently does nothing.

Expected: middle-click closes the tab, matching standard browser and editor tab behavior.

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Type: Bug. Priority: Low. Effort: Low. Labels: ui, confirmed-repro.

Report: https://get-bb.github.io/reports/issues/2756.html

Verdict: REPRODUCED. Confidence: high. Reproduction label: confirmed-repro.

Root cause: The shared tab component does not connect auxiliary button clicks to the existing close callback.

Fix: #2764. The focused test failed before the fix and passed after it.