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]>

packages/use-sync-external-store-esm/shim/index.js

@@ -0,0 +1 @@
+export { useSyncExternalStore } from "../index.js";

packages/use-sync-external-store-esm/shim/with-selector.js

@@ -0,0 +1 @@
+export { useSyncExternalStoreWithSelector } from "../with-selector.js";