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.

time_tracker.py

@@ -12,13 +12,7 @@ import calendar
 import re
 from collections import defaultdict
 
-# Global drag state
-drag_info = {
-    'active': False,
-    'mode': None,  # 'paint' or 'erase'
-    'start_row': None,
-    'last_cell': None
-}
+
 
 def sanitize_csv_text(text):
     """Sanitize text for safe CSV writing"""
@@ -169,10 +163,11 @@ def validate_input(input_type, value, **kwargs):
         return sanitize_csv_text(value)
 
 class ClickableCell(tk.Frame):
-    def __init__(self, parent, row_col_key, callback, width=5, height=2, start_hour=9):
+    def __init__(self, parent, row_col_key, callback, time_tracker, width=5, height=2, start_hour=9):
         super().__init__(parent, relief="solid", borderwidth=1, width=width, height=height)
         self.row_col_key = row_col_key
         self.callback = callback
+        self.time_tracker = time_tracker
         self.checked = False
         self.start_hour = start_hour
         
@@ -196,12 +191,11 @@ class ClickableCell(tk.Frame):
         self.bind("<Button-1>", self.on_mouse_down)
 
     def on_mouse_down(self, event):
-        global drag_info
 
         # Start drag mode
-        drag_info['active'] = True
-        drag_info['mode'] = 'paint' if not self.checked else 'erase'
-        drag_info['start_row'] = self.row_col_key[0]
+        self.time_tracker.drag_info['active'] = True
+        self.time_tracker.drag_info['mode'] = 'paint' if not self.checked else 'erase'
+        self.time_tracker.drag_info['start_row'] = self.row_col_key[0]
 
         # Toggle this cell
         self.checked = not self.checked
@@ -210,13 +204,11 @@ class ClickableCell(tk.Frame):
         else:
             self.label.config(bg=self.default_bg, text=" ")
         self.callback(self.row_col_key, self.checked)
-        drag_info['last_cell'] = self.row_col_key
+        self.time_tracker.drag_info['last_cell'] = self.row_col_key
 
     def apply_drag_state(self, force_mode=None):
-        """Apply drag state to this cell"""
-        global drag_info
 
-        mode = force_mode or drag_info['mode']
+        mode = force_mode or self.time_tracker.drag_info['mode']
 
         if mode == 'paint' and not self.checked:
             self.checked = True
@@ -252,6 +244,14 @@ class TimeTracker:
         self.work_hours = settings['work_hours']
         self.archive_path = settings['archive_path']
 
+        # Drag state - moved from global to class attribute
+        self.drag_info = {
+            'active': False,
+            'mode': None,  # 'paint' or 'erase'
+            'start_row': None,
+            'last_cell': None
+        }
+
         # Main container with scrollbars
         main_container = tk.Frame(root)
         main_container.pack(fill=tk.BOTH, expand=True)
@@ -484,9 +484,8 @@ class TimeTracker:
 
     def on_global_drag(self, event):
         """Global drag handler that finds which cell we're over"""
-        global drag_info
 
-        if not drag_info['active']:
+        if not self.drag_info['active']:
             return
 
         # Find which widget we're currently over
@@ -501,19 +500,18 @@ class TimeTracker:
                 break
             current_widget = current_widget.master
 
-        if cell and cell.row_col_key != drag_info['last_cell']:
+        if cell and cell.row_col_key != self.drag_info['last_cell']:
             applied = cell.apply_drag_state()
             if applied:
-                drag_info['last_cell'] = cell.row_col_key
+                self.drag_info['last_cell'] = cell.row_col_key
 
     def on_global_up(self, event):
         """Global mouse up handler"""
-        global drag_info
 
-        drag_info['active'] = False
-        drag_info['mode'] = None
-        drag_info['start_row'] = None
-        drag_info['last_cell'] = None
+        self.drag_info['active'] = False
+        self.drag_info['mode'] = None
+        self.drag_info['start_row'] = None
+        self.drag_info['last_cell'] = None
 
     def create_headers(self):
         headers = ["Job", "Task Name", "Notes", "Customer"]
@@ -578,7 +576,7 @@ class TimeTracker:
         self.time_cells[row_num] = {}
         time_slots = self.work_hours * 4  # Calculate based on current settings
         for i in range(time_slots):
-            cell = ClickableCell(self.scrollable_frame, (row_num, i), self.on_time_cell_clicked, width=5, height=1, start_hour=self.start_hour)
+            cell = ClickableCell(self.scrollable_frame, (row_num, i), self.on_time_cell_clicked, self, width=5, height=1, start_hour=self.start_hour)
             cell.grid(row=row_num, column=4 + i, sticky="nsew", padx=1, pady=1)
             self.time_cells[row_num][i] = cell