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/src/module-display-routes.ts

@@ -6,6 +6,7 @@ import {
   getAuthorizedPrincipal,
   requirePermission,
 } from "./authorization-middleware.js";
+import { etagMatches } from "./conditional-request.js";
 import { createDisplayHostAssets, type DisplayHostAssets } from "./display-host-assets.js";
 export {
   MODULE_DISPLAY_DESCRIPTOR_PREFIX,
@@ -100,20 +101,6 @@ function displayFailure(c: Context): Response {
   return c.json({ code: "AUTHORIZATION_EVALUATION_FAILED" }, 503);
 }
 
-/** RFC 9110 weak comparison for a GET/HEAD `If-None-Match` precondition. */
-function etagMatches(headerValue: string | undefined, etag: string): boolean {
-  if (headerValue === undefined) {
-    return false;
-  }
-  if (headerValue.trim() === "*") {
-    return true;
-  }
-  return headerValue.split(",").some((candidate) => {
-    const value = candidate.trim();
-    return (value.startsWith("W/") ? value.slice(2) : value) === etag;
-  });
-}
-
 /** Copies and validates injected assets before registering any route. Production's loader already
  * creates this exact shape, while this check keeps a test/composition caller from pairing mutable
  * bytes with a stale validator. */