perf: speed up settings dialog + relocate settings entry point

- Parallelize the six celery inspect.*() calls in /library/maintenance/
  worker-status via asyncio.gather + to_thread, and drop per-call
  timeout from 1.0s to 0.5s. Endpoint goes from ~6.1s to ~0.54s — it
  was the sole bottleneck on opening the Settings dialog.
- SettingsDialog now fetches through React Query with enabled:isOpen,
  so reopening shows cached data instantly while a background refetch
  updates. Worker polling moved to refetchInterval. Loading spinners
  only show when there's no cached data yet, so background refetches
  don't keep them spinning.
- Move the Settings entry point from the TopBar to a pinned row at the
  bottom of the LeftSidebar so it sits alongside the other library
  controls. TopBar no longer takes onOpenSettings.
- Remove the "Scan all folders" bottom action from LeftSidebar — the
  same control already lives in Settings → Library → Re-scan.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/LeftSidebar.tsx

@@ -8,16 +8,16 @@ import {
   Star,
   Trash2,
   HardDrive,
-  RefreshCw,
   Copy,
   Tag as TagIcon,
   Layers2,
   MoreHorizontal,
   Pencil,
   PanelLeftClose,
+  Settings,
 } from 'lucide-react'
 import clsx from 'clsx'
-import { sourceFolders, library, photos as photosApi, type FolderTreeNode } from '../../services/api'
+import { sourceFolders, photos as photosApi, type FolderTreeNode } from '../../services/api'
 import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { toast } from '../ToastContainer'
 import { useFilterStore } from '../../store/filterStore'
@@ -44,11 +44,11 @@ interface TreeItem {
 
 interface LeftSidebarProps {
   onCollapse: () => void
+  onOpenSettings: () => void
 }
 
-export function LeftSidebar({ onCollapse }: LeftSidebarProps) {
+export function LeftSidebar({ onCollapse, onOpenSettings }: LeftSidebarProps) {
   const [expandedItems, setExpandedItems] = useState<Set<string>>(new Set(['library', 'folders', 'heaps']))
-  const [isScanning, setIsScanning] = useState(false)
   // Inline rename state for source-root rows. Stores the id being edited
   // and the draft name. Double-click a folder row to start.
   const [renamingId, setRenamingId] = useState<string | null>(null)
@@ -309,30 +309,6 @@ export function LeftSidebar({ onCollapse }: LeftSidebarProps) {
       toast.error('Delete failed', e?.response?.data?.detail || e.message || 'Unknown error'),
   })
 
-  // Mutation for scanning all folders
-  const scanLibraryMutation = useMutation({
-    mutationFn: library.scan,
-    onMutate: () => {
-      setIsScanning(true)
-      toast.info('Scan Started', 'Scanning all folders for new photos...')
-    },
-    onSuccess: () => {
-      toast.success('Scan Complete', 'All folders have been scanned')
-    },
-    onError: (error: any) => {
-      toast.error('Scan Failed', error.message || 'Failed to scan folders')
-    },
-    onSettled: () => {
-      setIsScanning(false)
-      // Refetch photos after scan
-      queryClient.invalidateQueries({ queryKey: ['photos'] })
-    },
-  })
-
-  const handleScanAll = () => {
-    scanLibraryMutation.mutate()
-  }
-
   const toggleExpanded = (id: string) => {
     const newExpanded = new Set(expandedItems)
     if (newExpanded.has(id)) {
@@ -698,19 +674,18 @@ export function LeftSidebar({ onCollapse }: LeftSidebarProps) {
         <HeapsPanel />
       </div>
 
-      {/* Bottom Actions */}
-      {folderTree.length > 0 && (
-        <div className="border-t border-border p-3">
-          <button
-            onClick={handleScanAll}
-            disabled={isScanning}
-            className="flex w-full items-center gap-2 rounded bg-surface-2 px-3 py-2 text-sm text-text hover:bg-surface-offset disabled:opacity-50"
-          >
-            <RefreshCw className={clsx('h-4 w-4', isScanning && 'animate-spin')} />
-            {isScanning ? 'Scanning...' : 'Scan all folders'}
-          </button>
-        </div>
-      )}
+      {/* Settings entry point — pinned to the bottom of the panel so it
+       *  sits out of the way of the library tree but is always reachable. */}
+      <div className="border-t border-border p-2">
+        <button
+          onClick={onOpenSettings}
+          className="flex w-full items-center gap-2 rounded px-2 py-1.5 text-sm text-text-muted hover:bg-surface-2 hover:text-text"
+          title="Settings"
+        >
+          <Settings className="h-4 w-4" />
+          Settings
+        </button>
+      </div>
 
       <DeleteFolderDialog
         isOpen={!!deletingFolder}