Pause TanStack interval refetches when the tab is hidden (D8)

Set refetchIntervalInBackground: false as a QueryClient default so all
interval-based polls (widgets ~30s, message-queue 5s, media build progress
1s) pause when document.visibilityState === 'hidden'. Battery-friendly on
mobile -- the dashboard is the page most likely to be left open on a phone.

The media build-progress poll previously forced refetchIntervalInBackground:
true; that override is removed so it inherits the default. The build keeps
running server-side; the poll resumes and catches up when the user returns
to the tab.

122 tests pass; lint/build green.

Refs openspec/changes/mobile-responsive-parity/verify-report.md residual
risk #3 (D8 battery follow-up).

frontend/src/hooks/useMedia.ts

@@ -14,7 +14,10 @@ export function useMediaStatus(jellyfinServiceId?: string) {
 		staleTime: 5_000,
 		refetchInterval: (query) =>
 			query.state.data?.build_running ? 1000 : false,
-		refetchIntervalInBackground: true,
+		// Inherit the default refetchIntervalInBackground: false — pause the
+		// 1s build-progress poll when the tab is hidden. The build keeps
+		// running server-side; the poll resumes and catches up on return.
+		// Battery-friendly (D8 follow-up).
 	});
 }