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]>

surfaces/shell/index.html

@@ -11,7 +11,9 @@
     <div id="ekklesia-host"></div>
     <!-- A real module script, from the same origin, with no inline script anywhere on the page:
          a deployment can therefore apply a strict script-src without exceptions, and the runtime
-         `import()` of a published module artifact stays same-origin under it. -->
-    <script src="./app.js" type="module"></script>
+         `import()` of a published module artifact stays same-origin under it. The path is absolute
+         and fixed, exactly like the display host's, so it names one host-owned route rather than a
+         file relative to whatever URL served the page. -->
+    <script src="/assets/ekklesia-dashboard/app.js" type="module"></script>
   </body>
 </html>