Disable the command palette

It's not used for anything useful in Frogmouth right now, and will also
bypass the dark/light mode sticky toggle too. This can be fixed (the code to
make that sticky pre-dates the command palette by quite some way) but given
there's no other use for the command palette at the moment I'll just disable
it.

It can come back when actual commands get implemented.

frogmouth/app/app.py

@@ -18,6 +18,8 @@ class MarkdownViewer(App[None]):
     TITLE = APPLICATION_TITLE
     """The main title for the application."""
 
+    ENABLE_COMMAND_PALETTE = False
+
     def __init__(self, cli_args: Namespace) -> None:
         """Initialise the application.