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/utils/link-computer-refs.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, test } from "bun:test";
 import botRef from "./bot-ref";
+import workspaceRef from "./workspace-ref";
 import computerRef from "./computer-ref";
 import linkComputerRefs from "./link-computer-refs";
 import takeOverRef from "./take-over-ref";
@@ -34,4 +35,12 @@ describe("linkComputerRefs", () => {
             }),
         ).toBe("[bot 4](#bot-4) is on it. See [bot:x] and [bot:5](https://example.com).");
     });
+
+    test("links every workspace a message names", () => {
+        expect(
+            linkComputerRefs({
+                text: `Added to ${workspaceRef({ id: 2 })}. Not [workspace:x] or [workspace:3](https://example.com).`,
+            }),
+        ).toBe("Added to [workspace 2](#workspace-2). Not [workspace:x] or [workspace:3](https://example.com).");
+    });
 });