M1.5 frontend: admin Settings UI + config store + register gating

- config store (public /api/config: site_name, allow_registration, version),
  loaded in the router guard; site name drives the board header.
- session User gains is_admin.
- /settings route with requiresAdmin guard + admin-only gear link in the header;
  SettingsView renders grouped typed fields (text/number/toggle), saves via
  PATCH /api/settings with saved/error feedback, refreshes public config.
- Register link hidden + /register route blocked when registration is closed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/views/LoginView.vue

@@ -2,11 +2,13 @@
 import { ref } from "vue";
 import { useRouter, useRoute } from "vue-router";
 import { useSessionStore } from "../stores/session";
+import { useConfigStore } from "../stores/config";
 import BaseInput from "../components/BaseInput.vue";
 import BaseButton from "../components/BaseButton.vue";
 import type { ApiError } from "../api/client";
 
 const session = useSessionStore();
+const config = useConfigStore();
 const router = useRouter();
 const route = useRoute();
 
@@ -72,7 +74,7 @@ async function submit() {
         <BaseButton type="submit" :loading="loading">Sign in</BaseButton>
       </form>
 
-      <p class="mt-6 text-center text-sm text-neutral-500 dark:text-neutral-400">
+      <p v-if="config.allowRegistration" class="mt-6 text-center text-sm text-neutral-500 dark:text-neutral-400">
         No account?
         <RouterLink to="/register" class="font-semibold text-brand-700 hover:underline dark:text-brand"
           >Create one</RouterLink