Add auto-close registration and admin settings page

First user to register automatically becomes admin and registration
closes. Admins can re-open registration from a new settings page.
Uses Better Auth admin plugin for role management and a new appSettings
table for the registration flag. Mobile tab bar now links to settings
instead of inline logout.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/auth/client.ts

@@ -1,7 +1,10 @@
 "use client";
 
+import { adminClient } from "better-auth/client/plugins";
 import { createAuthClient } from "better-auth/react";
 
-export const authClient = createAuthClient();
+export const authClient = createAuthClient({
+  plugins: [adminClient()],
+});
 
 export const { signIn, signUp, signOut, useSession } = authClient;

lib/auth/server.ts

@@ -1,7 +1,14 @@
 import { betterAuth } from "better-auth";
 import { drizzleAdapter } from "better-auth/adapters/drizzle";
+import { APIError } from "better-auth/api";
+import { admin } from "better-auth/plugins";
 import { v4 as uuid } from "uuid";
 import { db } from "@/lib/db/client";
+import {
+  getUserCount,
+  isRegistrationOpen,
+  setSetting,
+} from "@/lib/services/settings";
 
 export const auth = betterAuth({
   database: drizzleAdapter(db, {
@@ -10,9 +17,40 @@ export const auth = betterAuth({
   emailAndPassword: {
     enabled: true,
   },
+  plugins: [admin()],
   advanced: {
     database: {
       generateId: () => uuid(),
     },
   },
+  databaseHooks: {
+    user: {
+      create: {
+        before: async (userData) => {
+          const userCount = await getUserCount();
+          if (userCount === 0) {
+            return {
+              data: {
+                ...userData,
+                role: "admin",
+              },
+            };
+          }
+          const open = await isRegistrationOpen();
+          if (!open) {
+            throw new APIError("FORBIDDEN", {
+              message: "Registration is currently closed",
+            });
+          }
+          return { data: userData };
+        },
+        after: async () => {
+          const userCount = await getUserCount();
+          if (userCount === 1) {
+            await setSetting("registrationOpen", "false");
+          }
+        },
+      },
+    },
+  },
 });