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/TopBar.tsx

@@ -1,8 +1,7 @@
-import { Settings, PanelLeftOpen, PanelRightOpen } from 'lucide-react'
+import { PanelLeftOpen, PanelRightOpen } from 'lucide-react'
 import muliLogo from '../../assets/muli-logo.png'
 
 interface TopBarProps {
-  onOpenSettings: () => void
   leftSidebarOpen: boolean
   rightSidebarOpen: boolean
   onExpandLeft: () => void
@@ -21,7 +20,6 @@ interface TopBarProps {
  * collapse twin lives in the panel's own header.
  */
 export function TopBar({
-  onOpenSettings,
   leftSidebarOpen,
   rightSidebarOpen,
   onExpandLeft,
@@ -54,13 +52,6 @@ export function TopBar({
             <PanelRightOpen className="h-4 w-4" />
           </button>
         )}
-        <button
-          onClick={onOpenSettings}
-          className="rounded p-1.5 text-text-muted transition-colors hover:bg-surface-2 hover:text-text"
-          title="Settings"
-        >
-          <Settings className="h-4 w-4" />
-        </button>
       </div>
     </header>
   )