Add interactive day selection to calendar

- Added click handlers to calendar days for user interaction
- Implemented selected day state tracking in calendar component
- Added CSS styling for selected days with green highlight
- Selected days show distinct visual feedback with borders and shadows
- Supports combination states (selected+today, selected+events)

🤖 Generated with [Claude Code](https://claude.ai/code)

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

index.html

@@ -311,6 +311,26 @@
             background: #e1f5fe;
         }
 
+        .calendar-day.selected {
+            background: #e8f5e8;
+            border: 2px solid #4caf50;
+            box-shadow: 0 0 8px rgba(76, 175, 80, 0.3);
+        }
+
+        .calendar-day.selected.has-events {
+            background: #f1f8e9;
+        }
+
+        .calendar-day.selected.today {
+            background: #e0f2f1;
+            border: 2px solid #4caf50;
+        }
+
+        .calendar-day.selected .day-number {
+            color: #2e7d32;
+            font-weight: 700;
+        }
+
         .day-number {
             font-weight: 600;
             font-size: 1.1rem;