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>

drizzle/20260301184938_lazy_aaron_stack/migration.sql

@@ -0,0 +1,10 @@
+CREATE TABLE `appSettings` (
+	`key` text PRIMARY KEY,
+	`value` text
+);
+--> statement-breakpoint
+ALTER TABLE `session` ADD `impersonatedBy` text;--> statement-breakpoint
+ALTER TABLE `user` ADD `role` text DEFAULT 'user';--> statement-breakpoint
+ALTER TABLE `user` ADD `banned` integer DEFAULT false;--> statement-breakpoint
+ALTER TABLE `user` ADD `banReason` text;--> statement-breakpoint
+ALTER TABLE `user` ADD `banExpires` integer;