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/playlists/playlists_provider.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:drift/drift.dart' as drift;
 import 'package:flutter/foundation.dart' show debugPrint;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -123,29 +125,45 @@ final playlistDetailProvider =
         tracks: const [],
       );
 
+  Future<bool> isOnline() async {
+    try {
+      return await ref
+          .read(connectivityProvider.future)
+          .timeout(const Duration(seconds: 3), onTimeout: () => true);
+    } catch (_) {
+      return true;
+    }
+  }
+
+  Future<void> fetchAndPopulate() async {
+    final api = await ref.read(playlistsApiProvider.future);
+    final fresh = await api.get(id);
+    await db.batch((b) {
+      b.insert(db.cachedPlaylists, fresh.playlist.toDrift(),
+          mode: drift.InsertMode.insertOrReplace);
+      for (var i = 0; i < fresh.tracks.length; i++) {
+        final t = fresh.tracks[i];
+        if (t.trackId == null) continue;
+        b.insert(
+          db.cachedPlaylistTracks,
+          CachedPlaylistTracksCompanion.insert(
+            playlistId: id,
+            trackId: t.trackId!,
+            position: drift.Value(i),
+          ),
+          mode: drift.InsertMode.insertOrReplace,
+        );
+      }
+    });
+  }
+
+  var revalidated = false;
+
   await for (final playlistRows in playlistQuery.watch()) {
     if (playlistRows.isEmpty) {
-      if (await ref.read(connectivityProvider.future)) {
+      if (await isOnline()) {
         try {
-          final api = await ref.read(playlistsApiProvider.future);
-          final fresh = await api.get(id);
-          await db.batch((b) {
-            b.insert(db.cachedPlaylists, fresh.playlist.toDrift(),
-                mode: drift.InsertMode.insertOrReplace);
-            for (var i = 0; i < fresh.tracks.length; i++) {
-              final t = fresh.tracks[i];
-              if (t.trackId == null) continue;
-              b.insert(
-                db.cachedPlaylistTracks,
-                CachedPlaylistTracksCompanion.insert(
-                  playlistId: id,
-                  trackId: t.trackId!,
-                  position: drift.Value(i),
-                ),
-                mode: drift.InsertMode.insertOrReplace,
-              );
-            }
-          });
+          await fetchAndPopulate();
           // watch() re-emits next iteration with populated row.
         } catch (_) {
           yield emptyDetail();
@@ -177,6 +195,16 @@ final playlistDetailProvider =
     }).toList();
 
     yield PlaylistDetail(playlist: playlist, tracks: tracks);
+
+    // SWR: first complete cache hit kicks one background refresh so
+    // the playlist catches up to server state (new tracks, renames,
+    // etc.) without making the user wait.
+    if (!revalidated) {
+      revalidated = true;
+      if (await isOnline()) {
+        unawaited(fetchAndPopulate().catchError((_) {}));
+      }
+    }
   }
 });