fix(build): include assets in standalone output

With output: "standalone", next build writes server.js under
.next/standalone but leaves generated static/public assets in the
project root, so starting the standalone server directly (e.g. via PM2)
404s on JS/CSS/font/favicon requests and /login stays stuck loading.
Add a postbuild step that copies .next/static and public into the
standalone directory, skipping the workspace-traced CLI build which
already copies its own assets.

tests/unit/standalone-assets.test.js

@@ -0,0 +1,55 @@
+import { mkdirSync, readFileSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { mkdtempSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+import { copyStandaloneAssets } from "../../scripts/copy-standalone-assets.mjs";
+
+function createBuildFixture(distDir) {
+  const projectRoot = mkdtempSync(join(tmpdir(), "9router-standalone-assets-"));
+  const buildRoot = join(projectRoot, distDir);
+  mkdirSync(join(buildRoot, "standalone"), { recursive: true });
+  mkdirSync(join(buildRoot, "static", "chunks"), { recursive: true });
+  mkdirSync(join(projectRoot, "public"), { recursive: true });
+  writeFileSync(join(buildRoot, "static", "chunks", "app.js"), "static asset");
+  writeFileSync(join(projectRoot, "public", "favicon.svg"), "public asset");
+  return projectRoot;
+}
+
+describe("standalone build assets", () => {
+  it("copies static and public assets into the default standalone layout", () => {
+    const projectRoot = createBuildFixture(".next");
+
+    copyStandaloneAssets({ projectRoot, distDir: ".next" });
+
+    expect(readFileSync(join(projectRoot, ".next", "standalone", ".next", "static", "chunks", "app.js"), "utf8"))
+      .toBe("static asset");
+    expect(readFileSync(join(projectRoot, ".next", "standalone", "public", "favicon.svg"), "utf8"))
+      .toBe("public asset");
+  });
+
+  it("uses a custom Next dist directory", () => {
+    const projectRoot = createBuildFixture(".next-cli-build");
+
+    copyStandaloneAssets({ projectRoot, distDir: ".next-cli-build" });
+
+    expect(readFileSync(join(projectRoot, ".next-cli-build", "standalone", ".next-cli-build", "static", "chunks", "app.js"), "utf8"))
+      .toBe("static asset");
+  });
+
+  it("does not modify workspace-traced CLI builds", () => {
+    const projectRoot = createBuildFixture(".next-cli-build");
+    const previousMode = process.env.NEXT_TRACING_ROOT_MODE;
+    process.env.NEXT_TRACING_ROOT_MODE = "workspace";
+
+    try {
+      copyStandaloneAssets({ projectRoot, distDir: ".next-cli-build" });
+    } finally {
+      if (previousMode === undefined) delete process.env.NEXT_TRACING_ROOT_MODE;
+      else process.env.NEXT_TRACING_ROOT_MODE = previousMode;
+    }
+
+    expect(() => readFileSync(join(projectRoot, ".next-cli-build", "standalone", ".next-cli-build", "static", "chunks", "app.js")))
+      .toThrow();
+  });
+});