feat(flutter): SWR everywhere + nav hydration + cold-start home skeleton

Three changes addressing the cold-start spinner + stale-on-revisit pain.

A. SWR on remaining cacheFirst providers
- artistProvider, artistAlbumsProvider, artistTracksProvider all gain
  alwaysRefresh: true. Cache hit still renders instantly; one
  background refresh per subscription keeps the row from going stale
  forever.
- albumProvider (inline async*) and playlistDetailProvider (inline
  async*) now keep a `revalidated` flag and kick a one-shot
  background fetch on the first complete cache hit. Same effect as
  cacheFirst's alwaysRefresh, just inline.
- Three providers gained the connectivity timeout that
  album/artist/playlist already had.

B. Navigation hydration
- AlbumDetailScreen accepts `AlbumRef? seed`; ArtistDetailScreen
  accepts `ArtistRef? seed`. When the live provider is still loading,
  the seed populates cover/title/artist immediately so the page
  isn't blank.
- Routing wires `extra: AlbumRef|ArtistRef` from go_router into
  the seed parameter.
- Call sites updated: home (Recently added, Rediscover albums +
  artists, Last played), artist detail album grid. Where a ref isn't
  available (track actions sheet), the screen falls back to the
  spinner — no regression.

C. Cold-start home skeleton
- Replace the full-screen CircularProgressIndicator on /home with a
  layout-preserving skeleton: 5 section titles + 6 grey card-shaped
  placeholders per row. The page feels populated immediately;
  sections fill in independently as data arrives via the per-section
  providers.
- Drops the unused DelayedLoading import.

Net effect: re-visits to detail screens render instantly (cache hit
+ silent refresh); first visit from a tile shows the seed header
immediately while tracks load; cold-start home shows a layout
skeleton instead of a 30s blank spinner.

flutter_client/lib/shared/routing.dart

@@ -7,6 +7,8 @@ import '../auth/login_screen.dart';
 import '../auth/server_url_screen.dart';
 import '../library/album_detail_screen.dart';
 import '../library/artist_detail_screen.dart';
+import '../models/album.dart';
+import '../models/artist.dart';
 import '../discover/discover_screen.dart';
 import '../library/home_screen.dart';
 import '../library/library_screen.dart';
@@ -89,11 +91,19 @@ GoRouter buildRouter(Ref ref) {
           GoRoute(path: '/home', builder: (_, __) => const HomeScreen()),
           GoRoute(
             path: '/artists/:id',
-            builder: (_, s) => ArtistDetailScreen(id: s.pathParameters['id']!),
+            // `extra` carries an optional ArtistRef so the detail
+            // header renders immediately while tracks/albums load.
+            builder: (_, s) => ArtistDetailScreen(
+              id: s.pathParameters['id']!,
+              seed: s.extra is ArtistRef ? s.extra as ArtistRef : null,
+            ),
           ),
           GoRoute(
             path: '/albums/:id',
-            builder: (_, s) => AlbumDetailScreen(id: s.pathParameters['id']!),
+            builder: (_, s) => AlbumDetailScreen(
+              id: s.pathParameters['id']!,
+              seed: s.extra is AlbumRef ? s.extra as AlbumRef : null,
+            ),
           ),
           GoRoute(path: '/queue', builder: (_, __) => const QueueScreen()),
           GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),