Unify markdown on one engine (Milkdown Crepe)

Replace the two-engine setup (MDXEditor for editing + react-markdown for
display) with a single Crepe-based component used for both. Editing is inline
WYSIWYG with full GFM that actually works — task lists, tables, and code
blocks via type-to-create / the slash menu — and read-only mode renders the
exact same way, so write and read can no longer drift (the root cause of the
heading/list/table/checkbox bugs).

- markdown-editor.tsx: Crepe wrapper supporting edit + readOnly, themed to the
  glass card via --crepe-* variable overrides (scoped to .milkdown so they win
  over frame-dark's own definitions).
- text-particle-view.tsx: render the message card with the read-only editor;
  drop the react-markdown component map.
- Remove now-unused deps: @mdxeditor/editor, react-markdown, remark-gfm,
  rehype-highlight, highlight.js.

The compose immersive mode (short, centered plain text) is unchanged.

https://claude.ai/code/session_019DU6V5z6Nr4Vu7b1fBDnq4

js/desktop/package.json

@@ -58,7 +58,7 @@
   "dependencies": {
     "@livekit/components-react": "^2.9.20",
     "@livekit/components-styles": "^1.2.0",
-    "@mdxeditor/editor": "^4.0.1",
+    "@milkdown/crepe": "^7.21.1",
     "@sentry/electron": "^7.11.0",
     "@sentry/react": "^10.54.0",
     "@tanstack/react-query": "^5.90.21",
@@ -66,7 +66,6 @@
     "clsx": "^2.1.1",
     "electron-squirrel-startup": "^1.0.1",
     "firebase": "^12.10.0",
-    "highlight.js": "^11.11.1",
     "livekit-client": "^2.18.0",
     "lucide-react": "^0.575.0",
     "next-themes": "^0.4.6",
@@ -74,11 +73,8 @@
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
     "react-error-boundary": "^6.1.1",
-    "react-markdown": "^10.1.0",
     "react-router-dom": "^7.13.0",
     "react-use": "^17.6.0",
-    "rehype-highlight": "^7.0.2",
-    "remark-gfm": "^4.0.1",
     "shadcn": "^3.8.5",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.5.0",