Replace the composer textarea with a Tiptap markdown editor

The composer now writes markdown with bold, italic, strike, code,
links, quotes and lists, from shortcuts, markdown typing or a bubble
over the selection. @ mentions a bot and # a workspace, shown as chips
and saved as [bot:ID] / [workspace:ID]. Text selected in a message can
be quoted into the composer. User messages render as markdown, and
replies tell the bot who and what the user mentioned.

Adds an ESM stand-in for use-sync-external-store, since bunext keeps
React external in page bundles and the CommonJS original requires it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/data/app-data.ts

@@ -291,6 +291,10 @@ export const AppData = {
      * Longest dictation the composer records before it stops on its own.
      */
     ComposerVoiceMaxMs: 5 * 60 * 1000,
+    /**
+     * Most bots or workspaces the composer's mention menu lists at once.
+     */
+    ComposerMentionMaxItems: 6,
     /**
      * Most of a bot message read aloud; the rest is cut.
      */