Fix: auto-scroll tab bar to active tab

[Gabko14]

Summary

• Wraps the tab bar ScrollView in a ScrollViewReader to enable programmatic scrolling
• Scrolls to the active tab on appear (fixes opening Ghostly with a far-right active tab)
• Scrolls to the active tab on change (fixes new tab creation and keyboard navigation with many tabs)

Test plan

• Create 15+ tabs, verify new tab is scrolled into view
• With many tabs, use Cmd+Shift+] / [ to navigate — verify tab scrolls into view
• Quit and reopen with a far-right tab active — verify it's visible on launch
• All existing tests pass

[claude]

Review: Approved

Summary: Clean implementation of auto-scroll behavior for the tab bar using standard SwiftUI patterns.

What's Good

1. Correct pattern usage: ScrollViewReader + .id() is the idiomatic SwiftUI approach for programmatic scrolling

2. Proper nil handling: Guard clauses correctly handle the optional activeTabId

3. Good UX decisions:

   • onAppear scroll has no animation (avoids jarring startup effect)
   • onChange scroll uses smooth .easeInOut(duration: 0.2) animation
   • .center anchor keeps active tab visible with context

4. Minimal change: Restructures only what's necessary, no over-engineering

Testing

Per project conventions (CLAUDE.md), UI scroll behavior can't be unit tested due to MenuBarExtra limitations. The manual test plan is thorough and appropriate for this change.

Minor Note

No issues, but for awareness: if activeTabId changes rapidly (e.g., holding down Cmd+Shift+]), animations will queue. This is standard SwiftUI behavior and should be fine in practice since the final position will be correct.

LGTM