fix: Mount header widget and add 'm' keybinding for toggle mode

- Header widget was created but never mounted in compose()
- Added 'm' keybinding for toggle_mode (switch markdown/HTML view)

src/mail/app.py

@@ -128,6 +128,7 @@ class EmailViewerApp(App):
         Binding("3", "focus_3", "Focus Envelopes Panel"),
         Binding("4", "focus_4", "Focus Main Content"),
         Binding("w", "toggle_main_content", "Toggle Message View Window"),
+        Binding("m", "toggle_mode", "Toggle Markdown/HTML"),
     ]
 
     BINDINGS.extend(