Add a search field that is automatically focused

### Problem

Bringing up the shortcut overlay shows you a window with a *bunch* of text which can introduce a significant cognitive overload. Although text is well spaced and consideration have been taken to ensure legibility, it can be hard / straining to scan the entire content of the overlay for the one shortcut you're looking for.

### Proposal

Add a search field that is automatically focused, so that immediately upon bringing up the overlay you can start typing for what you want to filter the amount of shortcuts presented to you (e.g. bring up the overlay and start typing "screenshot area" would show you "Select an area to grab" and "Copy an area to clipboard").

This could be especially useful / powerful when combined with #115, so that you could additionally search / filter not only system-wide shortcuts but application-specific ones

### Prior Art (Optional)

Mouseless (macOS) does that

iPadOS also offers search for their shortcut overlay (see the search icon on the left)

The issue names no files, tests, or entry points. Start by locating the shortcut overlay implementation and its UI entry point, then determine how shortcut filtering and automatic focus fit together; done means the overlay opens with the search field focused and typing shows matching shortcuts.

Written by the indexing model from the issue text.