feat(shell): give every Dashboard page a real URL

The Dashboard had one address. The API served the page only at `/`, both
reference proxies forwarded only `/`, and the open module lived in React
state, so a refresh went back to the first module, nothing could be
bookmarked, and Back left the Dashboard (DESIGN.md §2.7).

ADR 0057 makes the address bar the source of truth:

- `/` is a host-owned Overview, now the landing page; `/modules/<id>`
  mounts that module. An id the server's navigation does not list shows
  "This page isn't available" and loads nothing; any other Dashboard
  address is a host-owned "Page not found".
- `/modules`, `/settings`, and `/account` are reserved whole, from one
  list (`@ekklesia/shell/dashboard-routes`) that the shell's router and
  the API's page routes both read. The API serves the same frozen page,
  ETag, and security headers anywhere inside them; look-alikes and other
  methods stay the sanitized 404.
- A small host-owned history replaces the selected-module state, built in
  the entry point so it stays the only file touching `window`. Navigation
  items are real links; modified clicks are left to the browser. Focus
  moves to the page after an in-place change, and the tab is named after
  the page.
- Sign-in still happens in place with no return value (ADR 0034), so a
  deep link opens its own page after sign-in.
- Both reference Caddyfiles forward the new paths; the deploy smoke now
  reloads a module's address through the real proxy and requires it to
  mount again.

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

scripts/lib/deploy-smoke-client.mjs

@@ -88,6 +88,21 @@ async function edgeChecks() {
     "the Dashboard page names the API's origin, rendered at run time (ADR 0045)",
   );
 
+  // Every Dashboard page answers with that same page, so a refresh stays put (ADR 0057).
+  for (const deepLink of ["/modules/example", "/settings/displays/add", "/account"]) {
+    const deepPage = await fetch(`${dashboardOrigin}${deepLink}`);
+    check(
+      deepPage.status === 200 &&
+        deepPage.headers.get("etag") === page.headers.get("etag") &&
+        (await deepPage.text()) === html,
+      `GET ${deepLink} through the proxy is the same Dashboard page`,
+    );
+  }
+  check(
+    await refusedByProxy(`${dashboardOrigin}/settings-old`),
+    "a look-alike of a Dashboard namespace is refused by the proxy",
+  );
+
   const script = await fetch(`${dashboardOrigin}/assets/ekklesia-dashboard/app.js`);
   await script.arrayBuffer();
   check(script.status === 200, "GET /assets/ekklesia-dashboard/app.js is 200");
@@ -152,6 +167,10 @@ async function splitRoutingChecks() {
     "the Dashboard hostname refuses /display/*",
   );
   check(await refusedByProxy(`${apiOrigin}/`), "the API hostname serves nothing at /");
+  check(
+    await refusedByProxy(`${apiOrigin}/settings/displays`),
+    "the API hostname serves no Dashboard page either",
+  );
   check(
     await refusedByProxy(`${apiOrigin}/assets/ekklesia-dashboard/app.js`),
     "the API hostname refuses the Dashboard's script",