feat(userscript): GM_xmlhttpRequest transport, GM storage, metadata banner and build

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/build-userscript.ts

@@ -0,0 +1,14 @@
+// bun run build:userscript → dist/web/mbseed.user.js (served by our server at /mbseed.user.js).
+// Set MBSEED_PUBLIC_URL to add our server to @connect.
+import pkg from "../package.json";
+import { userscriptBanner } from "./userscript-banner";
+
+const result = await Bun.build({ entrypoints: ["src/userscript/main.ts"], target: "browser", format: "iife" });
+if (!result.success) {
+  for (const log of result.logs) console.error(log);
+  process.exit(1);
+}
+const code = await result.outputs[0]!.text();
+const out = userscriptBanner(pkg.version, process.env.MBSEED_PUBLIC_URL || undefined) + code;
+await Bun.write("dist/web/mbseed.user.js", out);
+console.log(`wrote dist/web/mbseed.user.js (${Math.round(out.length / 1024)} KB)`);