feat(flutter): metadata prefetcher — pre-warm drift for likely tap targets

Across-the-board sluggishness was every cold-cache tap doing one
network round trip while the user waits. SWR helps on re-visits but
the first time you tap a tile from home you eat the latency.

MetadataPrefetcher listens to homeProvider. When /api/home returns,
it fires fire-and-forget reads on albumProvider + artistProvider for
the top-N items in each home section (recently added, rediscover,
most played, last played). Each provider read triggers the existing
cold-cache path, which writes to drift. By the time the user
actually taps a tile, it's already a drift hit and the detail
screen renders instantly.

Cap N=8 per section (covers what's visible on a typical phone
without scrolling). Set spans dedupe across sections so popular
artists don't get fetched five times. Errors are swallowed — a
failed prefetch is silent and the tile falls back to its on-tap
fetch behavior.

Wired alongside the existing audio prefetcher in app.dart's
postFrameCallback.

flutter_client/lib/app.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import 'cache/metadata_prefetcher.dart';
 import 'cache/prefetcher.dart';
 import 'cache/sync_controller.dart';
 import 'shared/routing.dart';
@@ -27,6 +28,11 @@ class _MinstrelAppState extends ConsumerState<MinstrelApp> {
       // ignore: unawaited_futures
       ref.read(syncControllerProvider.notifier).sync();
       ref.read(prefetcherProvider);
+      // Metadata prefetcher: when /api/home returns, fire background
+      // albumProvider/artistProvider reads for the top-N items in
+      // each section so subsequent taps are drift hits, not network
+      // round trips.
+      ref.read(metadataPrefetcherProvider);
     });
   }