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/health-routes.test.ts

@@ -25,6 +25,7 @@ import {
   READINESS_DATABASE_TIMEOUT_MS,
 } from "../src/health-routes.js";
 import { createInMemoryDashboardStore } from "./support/in-memory-dashboard-store.js";
+import { testDashboardHostAssets } from "./support/dashboard-host-assets.js";
 import { emptyModuleConfiguration } from "./support/in-memory-module-configuration.js";
 import { emptyModuleLifecycle } from "./support/in-memory-module-lifecycle.js";
 import { emptyModuleStorage } from "./support/in-memory-module-storage.js";
@@ -91,6 +92,7 @@ function buildApp(
     moduleConfiguration: emptyModuleConfiguration(),
     moduleStorage: emptyModuleStorage(),
     dashboard: createInMemoryDashboardStore(),
+    dashboardHostAssets: testDashboardHostAssets(),
     health,
     runtime,
     trustedOrigins: [trustedOrigin],