frontend: add admin settings page for base URL configuration

Phase 2: Create admin UI for system configuration

- Add new /admin/settings route in frontend.go (protected by AdminMiddleware)
- Create admin_settings.templ with HTMX-powered form for base URL
- Add Settings link to admin sidebar navigation
- Admin settings form submits via HTMX to PUT /api/system/config
- Success message displays after save with updated form

The settings page allows admins to configure the base URL used for
device sync URLs, OPDS endpoints, and API access.

templates/admin_sidebar.templ

@@ -14,6 +14,9 @@ templ AdminSidebar(user User, currentPath string) {
 				<a href="/admin/library" class={activeClass(currentPath, "/admin/library")} style="color: var(--text-primary)">
 					📚 Library Management
 				</a>
+				<a href="/admin/settings" class={activeClass(currentPath, "/admin/settings")} style="color: var(--text-primary)">
+					⚙️ System Settings
+				</a>
 			</nav>
 		</div>
 	</aside>