The app crashed when displaying code blocks with language identifiers
not bundled in Shiki (e.g. "env" from .env file edits). The language
string was cast to SupportedLanguages without validation, causing
resolveLanguage to throw an unhandled error that killed the thread.

Three layers of defense added:
- getHighlighterPromise falls back to "text" when Shiki rejects
- codeToHtml call wrapped in try/catch with "text" fallback
- CodeHighlightErrorBoundary prevents component tree crashes

Closes #266

Co-authored-by: Ahmed R <Ahmed@Ahmeds-MacBook-Pro.local>