Add GitHub-style code preview + UE icon in context menu

- New CodeView: syntax-highlighted source (highlight.js, bundled offline)
  with line numbers for new files, colored unified diff for edits. Covers
  cpp/c/h, cs, py, js/ts, json/uproject, xml, ini, glsl. hljs tokens mapped
  to the app palette so it's theme-aware. Replaces the bare DiffView.
- Context-menu items now support icons; the Launch Unreal Engine item shows
  the UE mark.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

package.json

@@ -15,6 +15,7 @@
     "@tauri-apps/plugin-opener": "^2",
     "@tauri-apps/plugin-process": "^2.3.1",
     "@tauri-apps/plugin-updater": "^2.10.1",
+    "highlight.js": "^11.11.1",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "three": "^0.185.1"