fix(web): let the IME keep Enter while composing

Android IMEs fire a keydown with key 'Enter' while a Chinese preedit is
still composing. The handler intercepted it unconditionally, so the IME
never got its commit key and sendText cleared input.value mid-composition,
which detaches the IME from the field - typing then shows nothing until
the input is refocused. Skip Enter during composition, and ignore the
stray Enter some IMEs emit right after compositionend.