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/src/auth-protocol.ts

@@ -277,3 +277,36 @@ export function readAuthLanding(search: string): HostAuthLanding | undefined {
   }
   return undefined;
 }
+
+/**
+ * Captures what an emailed verification or reset link asked for, then removes it from the address
+ * bar.
+ *
+ * A password-reset token is a bearer credential for one account. Leaving it in `location.search`
+ * would put it in the browser's history, in the tab title's shareable URL, and in the `Referer` of
+ * anything the page subsequently loaded. `history.replaceState` rewrites the entry in place rather
+ * than pushing a new one, so the Back button cannot return to the URL that held it. It is
+ * deliberately *not* stored anywhere afterwards — it lives only in React state for the lifetime of
+ * the reset form, and never in `localStorage` or `sessionStorage`.
+ *
+ * The location and history are passed in rather than read here, so this module still reads no
+ * global and `browser/main.tsx` remains the shell's one browser-global boundary. Only a recognized
+ * Ekklesia landing rewrites the URL; any other query is left exactly as it arrived.
+ */
+export function consumeAuthLanding(
+  location: Pick<Location, "pathname" | "search">,
+  history: Pick<History, "replaceState">,
+): HostAuthLanding | undefined {
+  const landing = readAuthLanding(location.search);
+  if (!landing) {
+    return undefined;
+  }
+  try {
+    history.replaceState(null, "", location.pathname);
+  } catch {
+    // A browser that refuses the rewrite (an exotic sandbox, a file: URL) must still be able to
+    // finish the flow: the token is already captured, and failing here would strand the person on a
+    // page that cannot do anything.
+  }
+  return landing;
+}