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/stores/config.ts

@@ -0,0 +1,38 @@
+import { defineStore } from "pinia";
+import { ref } from "vue";
+import { api } from "../api/client";
+
+interface PublicConfig {
+  site_name: string;
+  allow_registration: boolean;
+  version: string;
+}
+
+// Public, unauthenticated app config (site name, whether signups are open).
+export const useConfigStore = defineStore("config", () => {
+  const siteName = ref("ThoughtSync");
+  const allowRegistration = ref(true);
+  const version = ref("");
+  const loaded = ref(false);
+
+  async function load(): Promise<void> {
+    if (loaded.value) return;
+    try {
+      const cfg = await api.get<PublicConfig>("/api/config");
+      siteName.value = cfg.site_name;
+      allowRegistration.value = cfg.allow_registration;
+      version.value = cfg.version;
+    } catch {
+      // Keep defaults if the config endpoint is unreachable.
+    } finally {
+      loaded.value = true;
+    }
+  }
+
+  async function reload(): Promise<void> {
+    loaded.value = false;
+    await load();
+  }
+
+  return { siteName, allowRegistration, version, loaded, load, reload };
+});