feat: replace tab right-click close with hover × close button

[derrick4reals]

Summary

• Replace right-click (contextmenu) tab close with a visible × button on badge hover
• Close button appears on hover, clicks are isolated with stopPropagation/stopImmediatePropagation
• Adds aria-label="Close tab" for accessibility

Changes

• TabBar.renderBadge() now renders a × div instead of listening for contextmenu
• CSS: close button hidden by default, absolutely positioned, shown on badge hover

Test Plan

• Unit tests for close button rendering, canClose gating, click callback
• Manual: open 2+ tabs, hover over a tab badge, click × to close

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: fee9e8fc08

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

Hide close affordance when tab cannot actually close

Rendering the new × control whenever item.canClose is true exposes a clickable close action that can be a guaranteed no-op for users: in a single blank tab state, getTabBarItems() still sets canClose true, but TabManager.closeTab() immediately returns false for the lone empty tab (src/features/chat/tabs/TabManager.ts:317). With this commit, that mismatch becomes visible as a broken affordance (hover × appears, click does nothing), so the close button should only render when the tab is truly closable.

Useful? React with 👍 / 👎.

[derrick4reals]

Thanks for the review! This case is not reachable in practice: the tab bar is hidden when there is only a single tab (updateTabBarVisibility sets showTabBar = tabCount >= 2), so the × button is never visible for the lone-empty-tab state. No fix needed here.