[Quick Accent] Show a reference guide of the available characters across all languages

@daverayment is already working on this.

Since Apr 29, 2026.

### Description of the new feature / enhancement

It is not possible for a Quick Accent user to preview a character set's mappings without first enabling it in Settings and then trying out each key in turn, triggering the Quick Accent popup each time. The only alternative to this is to look at the source code, which is regularly updated with new languages or changed mappings.

This leads to users leaving the default "All available" character sets checkbox enabled, which includes all 40+ sets and results in overlong popup sizes for 'popular' keys such as E. The popup will likely contain characters which the user may never need. As we expand languages or introduce user-defined languages in the future, this will worsen.

This proposal is to add a "Reference Guide" for all character mappings across the available languages. This will allow users to see which character sets they actually require, and will include copy and filter capabilities for tracking down individual characters and/or copying them elsewhere without having to enable the character set beforehand.

### Scenario when this would be used?

In the Settings application when the user wants to inspect all character set mappings or find the sets which feature a specific character.

### Supporting information

_No response_