Allow disabling/overriding default keybindings in config

[jmserra]

description

Currently, when customizing keybindings in rainfrog_config.toml, there's no way to:

1. Disable default keybindings - Setting a keybinding to an empty string ("") causes a crash with error: enum Action does not have variant constructor
2. Hide overridden keybindings in UI - When overriding a default keybinding (e.g., changing Alt-1 to Ctrl-h for FocusMenu), the UI still displays the original Alt-1
   keybinding instead of showing the custom Ctrl-h binding.

proposed solution

• Setting a keybinding to "" should disable/unbind that key
• OR if a custom keybinding is defined for an action, the UI should display the custom binding instead of the default one

my use case:

I want to use Vim-style Ctrl+hjkl navigation instead of Alt+1-5, and want the UI to reflect my custom keybindings rather than showing the defaults.

Example Config:
[keybindings.Menu]
"" = "FocusMenu"
"" = "" # Should disable this, but causes crash