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/header.templ

@@ -122,11 +122,25 @@ templ Header(user User, currentPath string) {
 						}
 					</div>
 				</div>
-				<!-- Admin section (visible to admins only) -->
-				if user.Role == "admin" {
-					<div class="my-1.5 mx-3 border-t" style="border-color: color-mix(in srgb, var(--text-primary) 7%, transparent);"></div>
-					<p class="px-3 pb-1 text-xs font-medium uppercase tracking-wide" style="color: var(--text-secondary);">Administration</p>
-					<div class="space-y-0.5">
+			<!-- Admin section (visible to admins only) -->
+			if user.Role == "admin" {
+				<div x-data="{ adminOpen: window.location.pathname.startsWith('/admin') }" class="sidebar-panel">
+					<button
+						type="button"
+						@click="adminOpen = !adminOpen"
+						class="w-full flex items-center gap-3 px-3 py-2 rounded-lg text-sm font-medium transition-colors text-content-muted hover:text-content hover:bg-surface-hover"
+					>
+						@Icon("shield", "h-5 w-5 shrink-0")
+						<span>Administration</span>
+						<svg
+							class="h-4 w-4 ml-auto transition-transform"
+							:class="{ 'rotate-180': adminOpen }"
+							fill="none" stroke="currentColor" stroke-width="2" viewBox="0 0 24 24"
+						>
+							<path stroke-linecap="round" stroke-linejoin="round" d="M19 9l-7 7-7-7"/>
+						</svg>
+					</button>
+					<div x-show="adminOpen" x-cloak x-transition class="mt-1 space-y-0.5 pl-1">
 						<a href="/admin" class={ activeClass(currentPath, "/admin") }>
 							@Icon("grid", "h-5 w-5 shrink-0")
 							<span>Dashboard</span>
@@ -144,7 +158,8 @@ templ Header(user User, currentPath string) {
 							<span>Settings</span>
 						</a>
 					</div>
-				}
+				</div>
+			}
 			</div>
 		</aside>
 		<!-- Topbar -->