feat(api): serve the built Dashboard from the API origin

Serve surfaces/shell's built Dashboard from apps/api on the same origin
as /api (ADR 0044), exactly as the display host is served: a closed set
of files read once at startup, never a static directory, never a path
built from request input.

- The shell exports @ekklesia/shell/dashboard-assets (the exact built
  index.html and script URLs), emits the script at
  assets/ekklesia-dashboard/app.js, and index.html names that absolute
  path so dev and production serve the same URLs.
- GET / and GET /assets/ekklesia-dashboard/app.js answer from a frozen
  snapshot: no-cache with a strong ETag and 304 on match. Source maps
  are never served; every other path is the ordinary sanitized 404.
- The page sets nosniff, X-Frame-Options: DENY, Content-Security-Policy:
  frame-ancestors 'none' (that directive only), and Referrer-Policy:
  no-referrer, on 200 and 304 alike.
- A missing or empty shell build fails startup with a fixed
  DashboardHostAssetError before any module code is imported.
- The If-None-Match comparison moves to conditional-request.ts, shared
  with the display host routes.
- The emailed-link token stripping moves from browser/main.tsx into
  consumeAuthLanding (location and history injected) so it is tested;
  behavior is unchanged.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/api/tests/dashboard-host-assets.test.ts

@@ -0,0 +1,87 @@
+import { createHash } from "node:crypto";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const readFileMock = vi.hoisted(() => vi.fn());
+
+vi.mock("node:fs/promises", () => ({ readFile: readFileMock }));
+vi.mock("@ekklesia/shell/dashboard-assets", () => ({
+  dashboardHostAssetUrls: Object.freeze({
+    html: "file:///surface/browser/index.html",
+    script: "file:///surface/browser/assets/ekklesia-dashboard/app.js",
+  }),
+}));
+
+import {
+  createDashboardHostAssets,
+  DashboardHostAssetError,
+  loadDashboardHostAssets,
+} from "../src/dashboard-host-assets.js";
+
+function sha256Etag(text: string): string {
+  return `"${createHash("sha256").update(text).digest("base64url")}"`;
+}
+
+describe("Dashboard host asset loading", () => {
+  beforeEach(() => readFileMock.mockReset());
+
+  it("reads only the two declared file URLs in order and freezes one consistent snapshot", async () => {
+    const html = '<!doctype html><div id="ekklesia-host"></div>';
+    const script = "export const dashboardHost = true;\n";
+    readFileMock.mockResolvedValueOnce(html).mockResolvedValueOnce(script);
+
+    const assets = await loadDashboardHostAssets();
+
+    expect(readFileMock.mock.calls).toEqual([
+      [new URL("file:///surface/browser/index.html"), "utf8"],
+      [new URL("file:///surface/browser/assets/ekklesia-dashboard/app.js"), "utf8"],
+    ]);
+    expect(assets).toEqual({
+      html,
+      htmlEtag: sha256Etag(html),
+      script,
+      scriptEtag: sha256Etag(script),
+    });
+    expect(Object.isFrozen(assets)).toBe(true);
+  });
+
+  it("fails deterministically without attempting a neighboring file or exposing the read error", async () => {
+    readFileMock.mockRejectedValueOnce(
+      new Error("ENOENT /home/operator/private/surface-build secret-token"),
+    );
+
+    const failure = await loadDashboardHostAssets().catch((error: unknown) => error);
+
+    expect(readFileMock).toHaveBeenCalledTimes(1);
+    expect(failure).toBeInstanceOf(DashboardHostAssetError);
+    expect(failure).toMatchObject({
+      code: "DASHBOARD_HOST_ASSET_UNAVAILABLE",
+      message: "The Dashboard host browser assets are unavailable.",
+      name: "DashboardHostAssetError",
+    });
+    expect((failure as Error).stack).toBe(
+      "DashboardHostAssetError: The Dashboard host browser assets are unavailable.",
+    );
+    const publicText = `${String(failure)}\n${JSON.stringify(failure)}\n${(failure as Error).stack}`;
+    for (const secret of ["/home/", "operator", "ENOENT", "secret-token"]) {
+      expect(publicText).not.toContain(secret);
+    }
+  });
+
+  it("fails when the script is missing even though the page was read", async () => {
+    readFileMock
+      .mockResolvedValueOnce("<!doctype html>")
+      .mockRejectedValueOnce(new Error("ENOENT app.js"));
+
+    await expect(loadDashboardHostAssets()).rejects.toBeInstanceOf(DashboardHostAssetError);
+    expect(readFileMock).toHaveBeenCalledTimes(2);
+  });
+
+  it("rejects an empty page or script as an unavailable shell build", () => {
+    for (const [html, script] of [
+      ["", "export {};"],
+      ["<html></html>", "   \n"],
+    ] as const) {
+      expect(() => createDashboardHostAssets(html, script)).toThrow(DashboardHostAssetError);
+    }
+  });
+});