feat: Add ⇧⌘T shortcut to reopen closed tabs

[William-Laverty]

Summary

Implements #1656

This PR adds the ability to reopen recently closed editor tabs using the ⇧⌘T (Shift+Command+T) keyboard shortcut, a common convention in most editors and browsers.

Changes

• Add closedTabsHistory to Editor to track recently closed tabs (stores up to 20 tabs)
• Modify closeTab() to save tabs to the closed history before closing
• Add reopenLastClosedTab() method to Editor
• Add canReopenClosedTab computed property for future UI integration
• Add reopenClosedTab action to CodeEditWindowController
• Add Reopen Closed Tab menu item to File menu with ⇧⌘T shortcut

Behavior

• The closed tab history is maintained per-editor (each split view has its own history)
• Users can repeatedly press ⇧⌘T to reopen multiple tabs in the order they were closed
• If a file is already open, pressing ⇧⌘T will select it instead of creating a duplicate
• Maximum of 20 closed tabs are remembered to prevent excessive memory usage

Testing

1. Open CodeEdit with a workspace
2. Open several files in tabs
3. Close some tabs using ⌘W or the close button
4. Press ⇧⌘T to reopen them in reverse order

Related

Closes #1656