Add mod: lock-keys-notifier (#4531)

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## Changelog

A new mod that shows a highly customizable toast when a lock key (Caps/Num/Scroll/Insert) is toggled, displaying its new state.

## Mod authorship

If this pull request introduces a new mod, please complete the section below.

This mod was created by:

- - [ ] The submitter, without AI assistance
- - [ ] The submitter, with AI assistance
- - [X] Claude
- - [ ] ChatGPT
- - [ ] Gemini
- - [ ] Another AI (please specify): 
- - [ ] Other (please specify): 

Please select the options that best apply. Your selection does not affect the acceptance criteria, but it helps reviewers understand the context of the code and provide relevant feedback.

Author: havrlisan