refactor(admin): remove separate sidebar, integrate admin nav into main sidebar

- Delete AdminSidebar component entirely
- Add Administration section (Dashboard, Libraries, Users, Settings) to
  header sidebar, visible only for admin users
- Remove Admin Panel link from user dropdown
- Strip admin chrome (sidebar wrapper, back buttons) from all 5 admin
  page templates
- Fix activeClass to handle trailing-slash routes correctly
- Add isUserVisible helper for library visibility toggles
- Fix processing issues page: remove dead Alpine JS, wire HTMX dismiss
  with proper mediaItemId, add issue ID swap targets
- Add processing issue resolve/delete routes to library router
- Add GetProcessingIssueStatsData context-based method
- Fix users page: remove broken hx-headers auth, simplify role select
- Fix settings page: remove dead adminSettings Alpine ref

templates/header.templ

@@ -122,6 +122,29 @@ 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">
+						<a href="/admin" class={ activeClass(currentPath, "/admin") }>
+							@Icon("grid", "h-5 w-5 shrink-0")
+							<span>Dashboard</span>
+						</a>
+						<a href="/admin/library" class={ activeClass(currentPath, "/admin/library") }>
+							@Icon("library", "h-5 w-5 shrink-0")
+							<span>Libraries</span>
+						</a>
+						<a href="/admin/users" class={ activeClass(currentPath, "/admin/users") }>
+							@Icon("users", "h-5 w-5 shrink-0")
+							<span>Users</span>
+						</a>
+						<a href="/admin/settings" class={ activeClass(currentPath, "/admin/settings") }>
+							@Icon("gear", "h-5 w-5 shrink-0")
+							<span>Settings</span>
+						</a>
+					</div>
+				}
 			</div>
 		</aside>
 		<!-- Topbar -->
@@ -173,12 +196,6 @@ templ SidebarUserMenu(user User) {
 				@Icon("user", "h-4 w-4")
 				<span>Profile</span>
 			</a>
-			if user.Role == "admin" {
-				<a href="/admin" class="flex items-center gap-3 px-3 py-1.5 rounded-lg text-sm transition-colors hover:bg-surface-hover" style="color: var(--text-secondary);">
-					@Icon("settings", "h-4 w-4")
-					<span>Admin Panel</span>
-				</a>
-			}
 			<button
 				type="button"
 				@click="logout()"