Add context filter to Tasks TUI and fix calendar UI bugs

Tasks TUI:
- Add context support to TaskBackend interface (get_context, set_context,
  get_contexts methods)
- Implement context methods in DstaskClient
- Add Context section to FilterSidebar (above projects/tags)
- Context changes persist via backend CLI

Calendar TUI:
- Remove duplicate header from InvitesPanel (use border_title instead)
- Fix border_title color to use $primary
- Fix WeekGrid to always scroll to work day start (7am) on mount

src/calendar/widgets/WeekGrid.py

@@ -648,13 +648,10 @@ class WeekGrid(Vertical):
         current_row = (now.hour * rows_per_hour) + (now.minute // minutes_per_row)
         self.cursor_row = current_row
 
-        # Scroll to show work day start initially
+        # Always scroll to work day start initially (e.g., 7am)
         if self._body:
             work_start_row = config.work_day_start_hour() * rows_per_hour
-            # If current time is before work day start, scroll to work day start
-            # Otherwise scroll to show current time
-            scroll_target = min(work_start_row, current_row)
-            self._body.scroll_to(y=scroll_target, animate=False)
+            self._body.scroll_to(y=work_start_row, animate=False)
 
     def watch_week_start(self, old: date, new: date) -> None:
         """Handle week_start changes."""