Move drag_info from global to class attribute for better encapsulation

## Code Quality Improvements

### Global State Removal
- Eliminated global drag_info dictionary
- Moved drag_state management into TimeTracker class
- Removed all global drag_info dependencies

### Updated Components
- **ClickableCell constructor**: Added time_tracker parameter for proper reference
- **ClickableCell methods**: Updated to use self.time_tracker.drag_info
- **TimeTracker methods**: Updated on_global_drag() and on_global_up()
- **Instance creation**: Updated ClickableCell instantiation calls

### Benefits Achieved
- **Better Encapsulation**: State properly contained within class boundaries
- **Thread Safety**: Reduced race conditions from shared global state
- **Testability**: Individual instance testing now possible
- **Instance Isolation**: Multiple TimeTracker instances work independently
- **Maintainability**: Clearer code structure with explicit dependencies

### Verification
- ✅ All drag functionality preserved (paint/erase operations)
- ✅ Drag state management works correctly
- ✅ Multiple instances properly isolated
- ✅ All 6 existing test suites pass (no regressions)
- ✅ New comprehensive test suite created and passing
- ✅ Application starts and runs correctly

## Files Modified
- **time_tracker.py**: Global state removal and class attribute implementation
- **AGENTS.md**: Updated coding guidelines for class preferences
- **TODO.md**: Marked drag_info task as completed, updated progress
- **tests/test_drag_info_class_attribute.py**: New comprehensive test suite

## Testing
- Added complete test suite for drag_info functionality
- Tests verify global state removal and class attribute access
- Confirms multiple instance isolation
- Validates drag state management

Code quality significantly improved with zero functional regressions.

AGENTS.md

@@ -18,7 +18,7 @@
 - **Classes**: PascalCase (e.g., `ClickableCell`, `TimeTracker`)
 - **Functions/Methods**: snake_case (e.g., `load_settings`, `update_day_total`)
 - **Variables**: snake_case (e.g., `time_cells`, `data_rows`)
-- **Constants**: UPPER_SNAKE_CASE (e.g., `drag_info` global dict)
+- **Constants**: UPPER_SNAKE_CASE (e.g., class attributes instead of global variables)
 - **Private methods**: prefix with underscore (e.g., `_refresh_dropdowns`)
 
 ### Error Handling