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/services/dstask/client.py

@@ -349,3 +349,43 @@ class DstaskClient(TaskBackend):
         # This needs to run without capturing output
         result = self._run_command(["note", task_id], capture_output=False)
         return result.returncode == 0
+
+    def get_context(self) -> Optional[str]:
+        """Get the current context filter.
+
+        Returns:
+            Current context string, or None if no context is set
+        """
+        result = self._run_command(["context"])
+        if result.returncode == 0:
+            context = result.stdout.strip()
+            return context if context else None
+        return None
+
+    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
+        """
+        if context is None or context.lower() == "none" or context == "":
+            result = self._run_command(["context", "none"])
+        else:
+            result = self._run_command(["context", context])
+        return result.returncode == 0
+
+    def get_contexts(self) -> list[str]:
+        """Get available contexts based on tags.
+
+        For dstask, contexts are typically tag-based filters like "+work".
+        We derive available contexts from the existing tags.
+
+        Returns:
+            List of context strings (tag-based)
+        """
+        # Get all tags and convert to context format
+        tags = self.get_tags()
+        return [f"+{tag}" for tag in tags if tag]