feat: replace PrismJS with highlight.js for code syntax highlighting

PrismJS's CJS global state is fundamentally incompatible with Vite's ESM
dev mode, causing missing language support (~40% of dropdown languages)
and fragile loading hacks (optimizeDeps, global assignment).

Replace the entire Prism integration with highlight.js/lib/common which
is ESM-native, has no global state, and covers all 22 dropdown languages
out of the box (~37 common languages included).

Implementation:
- Custom CodeHighlightPlugin using hljs.highlight() for tokenization
- HTML parser extracts flat tokens from hljs output (handles nested spans)
- Token type mapping from hljs scopes to Lexical codeHighlight theme keys
- Selection preservation via absolute character offset save/restore
- Re-entrancy guard prevents infinite transform loops
- Removed @lexical/code-prism import and Vite optimizeDeps workaround

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

package.json

@@ -34,7 +34,6 @@
     "@lexical/headless": "0.42.0",
     "@tailwindcss/vite": "^4.2.2",
     "@types/node": "^25.5.0",
-    "@types/prismjs": "^1.26.6",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "5.1.0",
@@ -56,6 +55,7 @@
     "@lexical/react": "0.42.0",
     "@lexical/rich-text": "0.42.0",
     "@xyflow/react": "^12.10.1",
+    "highlight.js": "^11.11.1",
     "keytar": "^7.9.0",
     "lexical": "0.42.0",
     "lucide-react": "^0.577.0",