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/BoardView.vue

@@ -2,6 +2,7 @@
 import { computed, onMounted, ref, watch } from "vue";
 import { useRoute, useRouter } from "vue-router";
 import { useSessionStore } from "../stores/session";
+import { useConfigStore } from "../stores/config";
 import { useNotesStore, type Note, type NoteView } from "../stores/notes";
 import QuickAdd from "../components/QuickAdd.vue";
 import NoteCard from "../components/NoteCard.vue";
@@ -10,6 +11,7 @@ import BaseButton from "../components/BaseButton.vue";
 import Icon from "../components/Icon.vue";
 
 const session = useSessionStore();
+const config = useConfigStore();
 const notes = useNotesStore();
 const route = useRoute();
 const router = useRouter();
@@ -69,7 +71,7 @@ async function signOut() {
           <div class="flex h-8 w-8 items-center justify-center rounded-lg bg-brand text-sm font-black text-neutral-900">
             TS
           </div>
-          <span class="hidden font-semibold sm:inline">ThoughtSync</span>
+          <span class="hidden font-semibold sm:inline">{{ config.siteName }}</span>
         </div>
 
         <nav class="flex items-center gap-1">
@@ -89,6 +91,15 @@ async function signOut() {
         </nav>
 
         <div class="flex items-center gap-3">
+          <RouterLink
+            v-if="session.user?.is_admin"
+            to="/settings"
+            class="icon-btn"
+            title="Settings"
+            aria-label="Settings"
+          >
+            <Icon name="settings" />
+          </RouterLink>
           <span class="hidden text-sm text-neutral-500 sm:inline dark:text-neutral-400">{{
             session.user?.display_name
           }}</span>