[Feature] Use "key code" for the hotkeys, instead of the actual letters

### Is your feature request related to a problem? Please describe.

Currently, when you set a hotkey for navigation (`j`/`k`/etc.) — they are processed as actual letters of the alphabet. This creates problems for users using multiple different languages on their computers. E.g. Latin letter `j` and Cyrillic letter `о` are on the same key on the keyboard, but navigation in FreshRSS will work only for the former, when the currently active layout is the Latin-based one.

### Use case

Changing the behaviour will allow users not to care about the currently selected language, and they will always be able to navigate in FreshRSS.

### Describe the solution you’d like

Rely on `event.code` for the shortcuts, instead of the `event.key` as it currently is. E.g. both Latin `j` and Cyrillic `о` will trigger an event with `code="KeyJ"`.

### Describe alternatives you’ve considered

_No response_

### Additional context

_No response_