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/dialogs/SettingsDialog.tsx

@@ -13,16 +13,21 @@ import {
   CheckCircle2,
 } from 'lucide-react'
 import clsx from 'clsx'
+import { useQuery, useQueryClient } from '@tanstack/react-query'
 import {
   library,
-  type ThumbnailStats,
-  type LibraryStats,
   type MediaType,
-  type WorkerStatus,
-  type MissingStats,
 } from '../../services/api'
 import { toast } from '../ToastContainer'
 
+// React Query keys for the settings panels. Kept here (not in a shared
+// hook module) since they're internal to this dialog and used by the
+// runAction refresh step to invalidate after mutations.
+const SETTINGS_THUMB_STATS_KEY = ['settings', 'thumbnail-stats'] as const
+const SETTINGS_LIB_STATS_KEY = ['settings', 'library-stats'] as const
+const SETTINGS_WORKER_STATUS_KEY = ['settings', 'worker-status'] as const
+const SETTINGS_MISSING_STATS_KEY = ['settings', 'missing-stats'] as const
+
 interface SettingsDialogProps {
   isOpen: boolean
   onClose: () => void
@@ -40,70 +45,96 @@ interface SettingsDialogProps {
  * dialog opens or after any action completes.
  */
 export function SettingsDialog({ isOpen, onClose }: SettingsDialogProps) {
-  const [thumbStats, setThumbStats] = useState<ThumbnailStats | null>(null)
-  const [libStats, setLibStats] = useState<LibraryStats | null>(null)
-  const [workerStatus, setWorkerStatus] = useState<WorkerStatus | null>(null)
-  const [missingStats, setMissingStats] = useState<MissingStats | null>(null)
-  const [loadingStats, setLoadingStats] = useState(false)
-  const [loadingWorkers, setLoadingWorkers] = useState(false)
+  const queryClient = useQueryClient()
   const [showAllErrors, setShowAllErrors] = useState(false)
   // One key per action so each button has its own spinner without
   // blocking the others.
   const [busy, setBusy] = useState<Record<string, boolean>>({})
 
-  const refreshStats = useCallback(async () => {
-    setLoadingStats(true)
-    try {
-      const [thumbs, lib] = await Promise.all([
-        library.maintenance.thumbnailStats(),
-        library.stats(),
-      ])
-      setThumbStats(thumbs)
-      setLibStats(lib)
-    } catch (e) {
-      console.error('Failed to load settings stats', e)
-      toast.error('Could not load library stats')
-    } finally {
-      setLoadingStats(false)
-    }
-  }, [])
+  // All four panels fetch through React Query so cached data shows
+  // instantly on reopen while a background refetch updates the numbers.
+  // `enabled: isOpen` avoids firing requests while the dialog is closed,
+  // but the cache entries survive between opens (default gcTime = 5m).
+  const thumbStatsQuery = useQuery({
+    queryKey: SETTINGS_THUMB_STATS_KEY,
+    queryFn: library.maintenance.thumbnailStats,
+    enabled: isOpen,
+    // Treat as stale immediately so reopening the dialog triggers a
+    // background refetch on top of the cached view.
+    staleTime: 0,
+  })
+  const libStatsQuery = useQuery({
+    queryKey: SETTINGS_LIB_STATS_KEY,
+    queryFn: library.stats,
+    enabled: isOpen,
+    staleTime: 0,
+  })
+  // Worker status polls every 5s while the dialog is open — `refetchInterval`
+  // replaces the old setInterval loop. Missing-stats is relatively cheap
+  // but shares the same 5s rhythm to keep the orphan banner live.
+  const workerStatusQuery = useQuery({
+    queryKey: SETTINGS_WORKER_STATUS_KEY,
+    queryFn: library.maintenance.workerStatus,
+    enabled: isOpen,
+    refetchInterval: isOpen ? 5000 : false,
+    staleTime: 0,
+  })
+  const missingStatsQuery = useQuery({
+    queryKey: SETTINGS_MISSING_STATS_KEY,
+    queryFn: library.maintenance.missingStats,
+    enabled: isOpen,
+    refetchInterval: isOpen ? 5000 : false,
+    staleTime: 0,
+  })
 
-  const refreshWorkers = useCallback(async () => {
-    setLoadingWorkers(true)
-    try {
-      const [ws, ms] = await Promise.all([
-        library.maintenance.workerStatus(),
-        library.maintenance.missingStats(),
-      ])
-      setWorkerStatus(ws)
-      setMissingStats(ms)
-    } catch (e) {
-      console.error('Failed to load worker status', e)
-      toast.error('Could not load worker status')
-    } finally {
-      setLoadingWorkers(false)
-    }
-  }, [])
+  const thumbStats = thumbStatsQuery.data
+  const libStats = libStatsQuery.data
+  const workerStatus = workerStatusQuery.data
+  const missingStats = missingStatsQuery.data
+  // "loading" in the UI sense = fetching AND no cached data yet. Background
+  // refetches on top of cached data shouldn't flip the refresh spinners.
+  const loadingStats =
+    (thumbStatsQuery.isFetching && !thumbStatsQuery.data) ||
+    (libStatsQuery.isFetching && !libStatsQuery.data)
+  const loadingWorkers =
+    (workerStatusQuery.isFetching && !workerStatusQuery.data) ||
+    (missingStatsQuery.isFetching && !missingStatsQuery.data)
 
-  // Esc closes; load stats when opened. Workers section auto-polls
-  // every 5s while the dialog is open so the user sees live worker
-  // activity without manually hammering the refresh button.
+  const refreshStats = useCallback(() => {
+    queryClient.invalidateQueries({ queryKey: SETTINGS_THUMB_STATS_KEY })
+    queryClient.invalidateQueries({ queryKey: SETTINGS_LIB_STATS_KEY })
+  }, [queryClient])
+  const refreshWorkers = useCallback(() => {
+    queryClient.invalidateQueries({ queryKey: SETTINGS_WORKER_STATUS_KEY })
+    queryClient.invalidateQueries({ queryKey: SETTINGS_MISSING_STATS_KEY })
+  }, [queryClient])
+
+  // Surface fetch errors once (React Query de-dupes retries but we still
+  // want a single toast so the user knows something went wrong).
+  useEffect(() => {
+    if (!isOpen) return
+    if (thumbStatsQuery.error || libStatsQuery.error) {
+      console.error('Failed to load settings stats', thumbStatsQuery.error ?? libStatsQuery.error)
+      toast.error('Could not load library stats')
+    }
+  }, [isOpen, thumbStatsQuery.error, libStatsQuery.error])
+  useEffect(() => {
+    if (!isOpen) return
+    if (workerStatusQuery.error || missingStatsQuery.error) {
+      console.error('Failed to load worker status', workerStatusQuery.error ?? missingStatsQuery.error)
+      toast.error('Could not load worker status')
+    }
+  }, [isOpen, workerStatusQuery.error, missingStatsQuery.error])
+
+  // Esc closes.
   useEffect(() => {
     if (!isOpen) return
-    refreshStats()
-    refreshWorkers()
     const handler = (e: KeyboardEvent) => {
       if (e.key === 'Escape') onClose()
     }
     window.addEventListener('keydown', handler)
-    const poll = window.setInterval(() => {
-      refreshWorkers()
-    }, 5000)
-    return () => {
-      window.removeEventListener('keydown', handler)
-      window.clearInterval(poll)
-    }
-  }, [isOpen, onClose, refreshStats, refreshWorkers])
+    return () => window.removeEventListener('keydown', handler)
+  }, [isOpen, onClose])
 
   const runAction = useCallback(
     async <T,>(
@@ -117,7 +148,8 @@ export function SettingsDialog({ isOpen, onClose }: SettingsDialogProps) {
       try {
         const result = await fn()
         toast.success(successTitle, describe?.(result))
-        await Promise.all([refreshStats(), refreshWorkers()])
+        refreshStats()
+        refreshWorkers()
       } catch (e: unknown) {
         const message = e instanceof Error ? e.message : String(e)
         toast.error(`${successTitle} failed`, message)