Fix search stability and improve Escape key behavior

- Add bounds check in _mark_message_as_read to prevent IndexError
- Clear metadata_by_id when search returns no results
- Escape now focuses search input when in search mode instead of exiting
- Add focus_input() method to SearchPanel

src/mail/screens/SearchPanel.py

@@ -216,6 +216,11 @@ class SearchPanel(Widget):
         self._cancel_debounce()
         self.result_count = -1
 
+    def focus_input(self) -> None:
+        """Focus the search input field."""
+        input_widget = self.query_one("#search-input", Input)
+        input_widget.focus()
+
     @property
     def is_visible(self) -> bool:
         """Check if the panel is visible."""