fix: Correct HelpScreen instantiation

- Fix missing app_bindings argument
- Pass self.BINDINGS list to HelpScreen constructor
- Help screen now works without crash

src/mail/app.py

@@ -660,7 +660,7 @@ class EmailViewerApp(App):
 
     async def action_show_help(self) -> None:
         """Show help screen with keyboard shortcuts."""
-        help_screen = HelpScreen()
+        help_screen = HelpScreen(list(self.BINDINGS))
         self.push_screen(help_screen)
 
     def action_next(self) -> None: