Add custom TextMate code language support

[rossirpaulo]

Adds desktop support for importing self-contained TextMate JSON grammars and using their fence tags in both the editor and rendered Markdown.

Users can add, preview, enable, edit, reveal, and remove custom languages from Settings → Editor → Languages. Installed grammars are stored in the machine-wide ZenNotes config directory and update open editor and preview surfaces without changing note files.

The runtime keeps built-in language and diagram tags reserved, rejects grammars with unsupported external scope dependencies, bounds grammar and code-block sizes, and loads TextMate/Oniguruma only when needed. The Oniguruma WASM is embedded and decoded locally instead of fetched, so it works with Electron's CSP and packaged file URLs.

Validation completed:

• 58 shared-domain tests
• 804 app-core tests
• custom-language desktop persistence tests
• TypeScript checks for shared-domain, bridge-contract, app-core, web, and desktop
• web and desktop production builds

[rossirpaulo]

I'm adding this feature since I'm trying to move off Obsidian and I build a programming language. It's hard to move off if it means my snippets don't have custom highlighting. Hope this helps and gets merged. Lmk if you need help reviewing.

[rossirpaulo]

@adibhanna, any steps that I need to take to get this PR some momentum? lmk if there's anything not documented in CONTRIBUTING.md.