fix: Remove unused 'm' key binding from ContentContainer

- The 'm' key was repurposed for toggle view mode, causing conflict
- Removed Binding from ContentContainer to free up the key for other uses
- action_toggle_mode still exists for extensibility but has no keybinding
- Tests still passing

src/mail/widgets/ContentContainer.py

@@ -160,9 +160,7 @@ class ContentContainer(ScrollableContainer):
     # Reactive to track view mode and update UI
     current_mode: reactive[Literal["markdown", "html"]] = reactive("markdown")
 
-    BINDINGS = [
-        Binding("m", "toggle_mode", "Toggle View Mode"),
-    ]
+    BINDINGS = []
 
     def __init__(self, **kwargs):
         super().__init__(**kwargs)