fix(admin): expandable sidebar section + library manage toggle

- Admin section in sidebar now uses same expandable panel pattern as
  Appearance and User sections (toggle button with chevron, auto-opens
  when on /admin pages)
- Library Manage button toggles open/close instead of only opening
  (uses htmx.ajax for open, clears panel for close)

templates/admin_library.templ

@@ -142,6 +142,18 @@ templ AdminLibrary(user User, libraries []LibraryData, users []User) {
 				</div>
 			</div>
 			<script>
+				function toggleLibraryPanel(btn) {
+					var libId = btn.dataset.libId;
+					var panel = document.getElementById('library-panel-' + libId);
+					if (panel.innerHTML.trim() !== '') {
+						panel.innerHTML = '';
+					} else {
+						htmx.ajax('GET', '/admin/library/' + libId + '/panel', {
+							target: '#library-panel-' + libId,
+							swap: 'innerHTML'
+						});
+					}
+				}
 				function openEditModal(id, name, description) {
 					document.getElementById('edit-library-form').setAttribute('hx-put', '/admin/library/' + id);
 					document.getElementById('edit-library-name').value = name;
@@ -219,9 +231,8 @@ templ LibraryList(user User, libraries []LibraryData, users []User) {
 								Scan
 							</button>
 							<button
-								hx-get={ "/admin/library/" + library.ID + "/panel" }
-								hx-target={ "#library-panel-" + library.ID }
-								hx-swap="innerHTML"
+								data-lib-id={ library.ID }
+								onclick="toggleLibraryPanel(this)"
 								class="btn btn-secondary text-xs px-3 py-1.5"
 							>
 								@Icon("chevron-down", "h-4 w-4")