Mobile keyboard should not behave like IME

When editing text on a mobile keyboard, the edited text is highlighted as if it was being modified in IME mode. It should instead just behave like a regular input, which means we should _not_ enable IME if we're editing on a mobile keyboard.

One possible solution is to ignore the `composition` events when `is_mobile()` is `true`.

Start by tracing egui's mobile keyboard input handling and the composition events mentioned in the issue, then reproduce the highlighted-text behavior on a mobile keyboard. Done means mobile editing behaves like regular input without IME handling, while other composition-based input remains unaffected.

Written by the indexing model from the issue text.