Add mini-calendar sidebar to Calendar TUI

- Add MonthCalendar widget as a collapsible sidebar (toggle with 's')
- Sidebar syncs with main week grid (week highlight, selected date)
- Click dates in sidebar to navigate week grid to that date
- Click month navigation arrows to change displayed month
- Add goto_date() method to WeekGrid for date navigation

src/calendar/widgets/WeekGrid.py

@@ -761,3 +761,20 @@ class WeekGrid(Vertical):
         event = self.get_event_at_cursor()
         if event:
             self.post_message(self.EventSelected(event))
+
+    def goto_date(self, target_date: date) -> None:
+        """Navigate to a specific date.
+
+        Sets the week to contain the target date and places cursor on that day.
+        """
+        # Get the week start for the target date
+        week_start_date = get_week_start_for_date(target_date)
+
+        if self.week_start != week_start_date:
+            self.week_start = week_start_date
+
+        # Set cursor column to the target date
+        col = get_day_column_for_date(target_date, self.week_start)
+        if not self.include_weekends and col >= 5:
+            col = 4  # Last weekday if weekend
+        self.cursor_col = col