Allow customizing or disabling keyboard shortcuts

Buzz should allow users to customize or disable keyboard shortcuts.

I use Buzz on Linux with Fcitx5 and an IME. Some default shortcuts conflict with input methods:

- `Ctrl+;` opens the emoji picker
- `Ctrl+Space` is used for push-to-talk
- `Ctrl+J` is used by Buzz

For example, I use SKK, a Japanese input method. SKK uses keyboard-driven mode switching: one shortcut activates the input method, and another switches from direct Latin-character input to Japanese Hiragana input. The defaults above conflict with these operations.

This is not only a Japanese-language issue. Many users who write in languages not based solely on the Latin alphabet rely on IMEs with their own keyboard shortcuts—for example Chinese, Korean, and other input methods. Reserving common key combinations without an opt-out can make normal text entry difficult or impossible.

The current shortcut list appears to be read-only. Please consider:

- Allowing shortcuts to be rebound in Settings
- Allowing individual shortcuts, especially emoji picker and push-to-talk, to be disabled
- Clearly documenting all reserved shortcuts

This would improve accessibility and make Buzz easier to use with IMEs across different languages and platforms.