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/tasks/backend.py

@@ -269,3 +269,36 @@ class TaskBackend(ABC):
             True if successful
         """
         pass
+
+    @abstractmethod
+    def get_context(self) -> Optional[str]:
+        """Get the current context filter.
+
+        Returns:
+            Current context string, or None if no context is set
+        """
+        pass
+
+    @abstractmethod
+    def set_context(self, context: Optional[str]) -> bool:
+        """Set the context filter.
+
+        Args:
+            context: Context string (e.g., "+work", "project:foo") or None to clear
+
+        Returns:
+            True if successful
+        """
+        pass
+
+    @abstractmethod
+    def get_contexts(self) -> list[str]:
+        """Get available predefined contexts.
+
+        For taskwarrior, returns named contexts from config.
+        For dstask, may return common tag-based contexts.
+
+        Returns:
+            List of context names/filters
+        """
+        pass