On iOS: tap the share button in browser => add to home screen
On Android: tap the 3 dot menu button in browser => tap "install" or "add to home screen"
Edit: I lied, it's not directly from my inbox. NVM. I go to the commenter's comments and go to the context from there. The comment is usually first or second on their list, so it's not hard to find. But yeah, extra steps. I wish I could reply from the inbox in general.