feat(desktop): add visible close button to pane tabs

[angel12]

Summary

Add a conventional hover/focus × to closeable horizontal pane tabs, including file, artifact, and Browser previews.

Today these tabs can be closed through a context menu, middle-click, ⌘-click, or the focused-zone shortcut, but none of those paths is visible in the UI. In a preview such as an artifact source view, the visible toolbar only exposes actions like Copy and Download, so users reasonably conclude that the pane cannot be closed.

Implementation

• Reserve a fixed-size close slot so revealing the × never changes tab width.
• Reveal the close button on pointer hover and keyboard focus.
• Give the button the app's focus-visible ring because global styles suppress native outlines.
• Stop ordinary pointer/click propagation so closing cannot also activate or drag the tab.
• Preserve the established ⌘-click close gesture even when the pointer is directly over the new close button.
• Keep the existing middle-click and ⌘-click gestures on the rest of the tab.
• Preserve the dirty marker in the reserved slot; it yields visually to the × on hover/focus instead of disappearing permanently.
• Use the existing localized common.close label for the accessible name and tooltip.
• Keep vertical collapsed rails unchanged, where an inline × does not fit their writing mode.

Prior work / duplicate handling

This deliberately preserves Austin Pickett's original commit and authorship from #69392 by cherry-picking it rather than reimplementing the feature. That PR is currently unmergeable against main (mergeable_state: dirty) and has maintainer_can_modify: false, so it cannot be refreshed upstream. The follow-up commit resolves the outstanding keyboard-focus review note, preserves the dirty indicator, and includes the post-submission independent-review fix for meta-clicks over the new button.

This is complementary to #80958 / #86191, which address a separate persisted-hidden-header trap. This PR supplies the missing visible close affordance whenever the pane tab strip is present.

Supersedes #69392 while retaining its author credit.

Verification

• npx vitest run --project ui src/components/ui/pane-tab.test.tsx — 14 passed
• npm run test:ui — 513 files / 4,779 tests passed
• npm run check:lint — passed (typecheck clean; 0 lint errors, existing warnings only)
• npx prettier --check src/components/ui/pane-tab.tsx src/components/ui/pane-tab.test.tsx — passed
• git diff origin/main...HEAD --check — passed

Regression-test sabotage

With the new tests retained and PaneTab temporarily restored to origin/main, the focused test fails in four relevant cases: no accessible close button, no focus-visible close path, no retained dirty marker, and no clickable close action. Restoring the implementation returns the original suite to passing. A separate reviewer-discovered meta-click regression test failed against the first submitted revision (0 closes) and passes after the event-routing fix (1 close).

[angel12]

Post-submission independent review found one real interaction bug: the parent tab capture handler swallowed ⌘-clicks when the pointer was directly over the new ×. Added a differential regression test (0 closes before the fix, 1 after), exempted the close-button target from that parent suppression, and added keyboard-style activation coverage. Focused suite is now 14/14; typecheck, targeted lint, and formatting pass. CI has restarted on 28d9f54533.