Use inline WYSIWYG markdown editor for long text messages

Desktop: replace the Write/Preview tab toggle in the compose card with
MDXEditor, an inline WYSIWYG that renders markdown as you type (Obsidian/
Notion feel) and round-trips plain markdown, matching how particle content
is stored. Immersive mode is unchanged — short, plain notes still get the
centered large-type textarea, and ⌘+M still drops into the rich editor.
Shortcut handling moves to the capture phase so ⌘+Enter / Esc win over the
editor's own key handling.

Mobile: render markdown on the display side via react-native-marked's
useMarkdown hook (no FlatList, so it nests cleanly in the existing
ScrollView). Mirrors desktop's immersive-vs-card logic: short plain notes
stay centered; anything with markdown renders formatted instead of showing
raw syntax. Composer stays plain text.

https://claude.ai/code/session_019DU6V5z6Nr4Vu7b1fBDnq4

js/desktop/package.json

@@ -58,6 +58,7 @@
   "dependencies": {
     "@livekit/components-react": "^2.9.20",
     "@livekit/components-styles": "^1.2.0",
+    "@mdxeditor/editor": "^4.0.1",
     "@sentry/electron": "^7.11.0",
     "@sentry/react": "^10.54.0",
     "@tanstack/react-query": "^5.90.21",