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/message-preview.test.ts

@@ -12,6 +12,7 @@ describe("messagePreview", () => {
 
     test("names bots plainly", () => {
         expect(messagePreview({ text: "[bot:4] is on it." })).toBe("a bot is on it.");
+        expect(messagePreview({ text: "Moved to [workspace:2]." })).toBe("Moved to a workspace.");
     });
 
     test("keeps code text without its fences", () => {