UI: Detect when Secure Keyboard Input happen and tell the user

[huytd]

Apps that deal with keyboard input, like IMEs or Hammerspoon, usually suffer from losing access to all keyboard input when Secure Keyboard Input happens. This makes the user unable to type in Vietnamese, as well as not able to toggle the input mode using the shortcut keys.

Secure Keyboard Input is a macOS feature that will disable keyboard access for 3rd party apps when the user is in a private field like password input. See Apple's technical note on this topic for more details.

Fig's documentation has some note on how to deal with Secure Keyboard Input, and we should have the same thing https://fig.io/docs/support/secure-keyboard-input