Fixes:
- Theme now updates when switching macOS appearance while set to "system" (#20)
- Clickable URLs no longer include trailing punctuation from markdown links (#18)
- Language auto-detection on paste now works reliably — removed brittle custom heuristics in favour of highlight.js auto-detect restricted to supported languages
- Detected language now propagates to the syntax highlighter (was silently dropped before)
- Fixed thread safety issue between language detection and syntax highlighting