Add Monaco syntax highlighting to the diff panel

[evanklem]

What this does

Reuses the editor's Monaco tokenizer and polypore-warm theme to syntax-highlight the diff panel's lines instead of rendering flat text. The shared theme, language detection, and colorize helpers are factored into plugins/shared/monaco-highlight.ts and the editor is rewired onto them, so both panels share one source of truth. Diff-status tints (add/remove/change) stay dominant as the row background; token colors render over them.

How to test

1. npm run app, open the diff panel, and pick a changed source file from the rail (e.g. a .ts/.tsx/.css).
2. Confirm lines render with the same syntax colors as the editor, while the green/red/accent row tints still show the diff status.
3. Switch filesranch compare;highlighting tracks the active file's language.
4. Open a changsionless nameand confirm it falls back to uncolored text with no error5. Run npm rununplugins/shared/plugins/diff-hi.ts.## Checklist- [x] npm run- [x] cargo cl` passes (ifRust changed) —

• New behavxisting testsupdated