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]>

Dockerfile

@@ -16,6 +16,8 @@ WORKDIR /app
 RUN echo 'alias ll="ls -laF"' >/root/.bashrc
 
 COPY package.json bun.lock ./
+# Local packages the lockfile installs from (`file:` dependencies)
+COPY packages ./packages
 RUN bun install --frozen-lockfile
 
 COPY . /app/.