Implement custom layout persistence

Features:
- Custom layouts now saved to ~/.local/share/gridsnap/layouts.json
- Layouts automatically loaded on extension initialization
- Layouts persist across Cinnamon restarts and extension reloads
- Only custom layouts saved to file (default layouts remain in code)
- Added GLib and Gio imports for file operations
- Error handling for file read/write operations

Technical implementation:
- _loadLayouts(): Merges default layouts with saved custom layouts on startup
- _saveLayouts(): Extracts and saves only custom-* layouts to JSON file
- Automatically creates storage directory if it doesn't exist
- Called when user saves a layout in the graphical editor

Fixes TODO item #1 - Custom Layout Persistence

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

TODO.md

@@ -2,14 +2,14 @@
 
 ## Critical Issues
 
-### 1. Custom Layout Persistence (extension.js:297)
+### 1. Custom Layout Persistence ✅ COMPLETED
 **Priority: High**
-- [ ] Custom layouts are only stored in memory
-- [ ] Layouts are lost when extension reloads or Cinnamon restarts
-- [ ] Need to implement file-based storage
-- [ ] Location: `~/.config/gridsnap/layouts.json` or similar
-- [ ] Save layouts when created in graphical editor
-- [ ] Load layouts on extension initialization
+- [x] Custom layouts are only stored in memory
+- [x] Layouts are lost when extension reloads or Cinnamon restarts
+- [x] Need to implement file-based storage
+- [x] Location: `~/.local/share/gridsnap/layouts.json`
+- [x] Save layouts when created in graphical editor
+- [x] Load layouts on extension initialization
 
 ### 2. Remove Last Zone Implementation (extension.js:236-274)
 **Priority: Medium**