Implement split-based zone editor and multi-monitor support

Major changes:
- Replace free-form drawing with split-based tiling interface
- Add per-monitor zone configurations and storage
- Implement monitor selector UI for multi-monitor setups
- Add H/V keyboard shortcuts for horizontal/vertical splits
- Add divider dragging for adjusting zone positions
- Display real-time dimensions in center of each zone
- Migrate storage format to v2 with per-monitor layouts
- Update settings panel for per-monitor layout management

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

TODO.md

@@ -21,13 +21,13 @@
 
 ## Functionality Improvements
 
-### 3. Multi-Monitor Support
+### 3. Multi-Monitor Support ✅ COMPLETED
 **Priority: High**
-- [ ] Currently only works on primary monitor (extension.js:61)
-- [ ] Need per-monitor zone configurations
-- [ ] Detect which monitor window is on when snapping
-- [ ] Allow zone editor to work on any monitor
-- [ ] Store zone layouts per monitor
+- [x] Currently only works on primary monitor (extension.js:61)
+- [x] Need per-monitor zone configurations
+- [x] Detect which monitor window is on when snapping
+- [x] Allow zone editor to work on any monitor
+- [x] Store zone layouts per monitor
 
 ### 4. High-DPI Display Support
 **Priority: Medium**