feat: Slovak language support (sk-SK)

## Summary

Requesting Slovak (`sk-SK`) language support in Harper. As far as I can tell there is no existing issue or PR tracking Slovak specifically, so this is to track it. See also the general non-English roadmap in #2654.

## Why

Harper's speed, offline operation and privacy-first design make it very attractive for Slovak users — a niche currently served only by heavier, slower tools (LanguageTool / ltex-ls). There is no lightweight, offline grammar/spell checker for Slovak today.

## Open resources to seed it

- **Hunspell `sk_SK`** dictionary (`.aff` + `.dic`), widely packaged on Linux distros.
- **Slovak National Corpus** data and the **sk-spell** project (by Zdenko Podobný), which is also the basis of LanguageTool's Slovak spellchecker. Permissively licensed (LGPL / MPL).

## Notes on difficulty

Slovak is a West Slavic, strongly fusional language: rich nominal declension (6–7 cases × number × 3 genders, with animacy), extensive verb conjugation and aspect, and productive derivation. A plain word list won't be enough — solid morphological annotations / word-forms will matter. In terms of effort this is probably closer to the Belarusian attempt (#2195) than to the Romance/German PRs (#2150, #3402).

## How I can help

I'm happy to contribute on the non-Rust side — assembling `dictionary-sk` + annotations from the resources above. Since #2150 and #3402 are still converging on the multi-language framework, I'd appreciate guidance on the currently recommended branch/structure to target before I start.

Start by reading the multi-language work in #2150 and #3402 and confirm with maintainers which branch and structure to target. Then examine the Hunspell sk_SK dictionary, Slovak National Corpus data, and sk-spell resources for dictionary-sk and its annotations. Done means Slovak support is assembled within the recommended framework.

Written by the indexing model from the issue text.