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/MonthCalendar.py

@@ -240,3 +240,41 @@ class MonthCalendar(Widget):
         self.display_month = date(year, month, 1)
         self.post_message(self.MonthChanged(self.display_month))
         self.refresh()
+
+    def on_click(self, event) -> None:
+        """Handle mouse clicks on the calendar."""
+        # Row 0 is the month header (< Month Year >)
+        # Row 1 is day names (Mo Tu We ...)
+        # Row 2+ are the week rows
+        y = event.y
+        x = event.x
+
+        if y == 0:
+            # Month header - check for navigation arrows
+            if x < 2:
+                self.prev_month()
+            elif x >= self.size.width - 2:
+                self.next_month()
+            return
+
+        if y == 1:
+            # Day names row - ignore
+            return
+
+        # Week row - calculate which date was clicked
+        week_idx = y - 2
+        weeks = self._weeks
+        if week_idx < 0 or week_idx >= len(weeks):
+            return
+
+        week = weeks[week_idx]
+        # Each day takes 3 characters ("DD "), so find which day column
+        day_col = x // 3
+        if day_col < 0 or day_col >= 7:
+            return
+
+        clicked_date = week[day_col]
+        if clicked_date is not None:
+            self.selected_date = clicked_date
+            self.post_message(self.DateSelected(clicked_date))
+            self.refresh()

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