Refactor _removeLastZone to avoid recreating entire editor

Simplified the _removeLastZone() function to be more efficient:

Before:
- Removed last zone from array
- Destroyed and recreated entire editor interface (_cancelEditor + startEditor)
- Recreated all remaining zones from scratch
- Caused visual flicker and was inefficient
- Could leave dangling event listeners

After:
- Remove last zone from zones array
- Remove and destroy only the last zone actor
- No need to update zone numbers (removing last zone doesn't affect other numbers)
- No editor recreation needed

Benefits:
- No visual flicker
- Much more efficient (O(1) instead of O(n))
- Cleaner code (15 lines vs 50+ lines)
- No risk of dangling event listeners from recreation
- Better user experience

Fixes TODO item #2.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

TODO.md

@@ -11,13 +11,13 @@
 - [x] Save layouts when created in graphical editor
 - [x] Load layouts on extension initialization
 
-### 2. Remove Last Zone Implementation (extension.js:236-274)
+### 2. Remove Last Zone Implementation ✅ COMPLETED
 **Priority: Medium**
-- [ ] `_removeLastZone()` destroys and recreates entire editor interface
-- [ ] Causes visual flicker
-- [ ] Inefficient approach
-- [ ] Could leave dangling event listeners
-- [ ] Refactor to only remove the last zone widget without recreating everything
+- [x] `_removeLastZone()` destroys and recreates entire editor interface
+- [x] Causes visual flicker
+- [x] Inefficient approach
+- [x] Could leave dangling event listeners
+- [x] Refactor to only remove the last zone widget without recreating everything
 
 ## Functionality Improvements