feat(flutter/playlists): empty-state hint on empty playlist detail

When a playlist has zero tracks, the detail screen previously showed
just the header with no indication of what to do next. Adds an inline
hint mirroring the web's copy: "No tracks yet. Add some via the
\"Add to playlist…\" entry on any track row."

Implementation: itemCount goes from tracks.length + 1 to
(tracks.isEmpty ? 2 : tracks.length + 1) and the builder emits the
hint at index 1 when tracks is empty. Header still renders at index 0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/playlists/playlist_detail_screen.dart

@@ -51,6 +51,7 @@ class _Body extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
     final tracks = detail.tracks;
     final playable = tracks.where((t) => t.isAvailable).toList();
 
@@ -58,9 +59,22 @@ class _Body extends ConsumerWidget {
       onRefresh: () async =>
           ref.refresh(playlistDetailProvider(detail.playlist.id).future),
       child: ListView.builder(
-        itemCount: tracks.length + 1, // header + rows
+        // Header + (track rows | empty hint).
+        itemCount: tracks.isEmpty ? 2 : tracks.length + 1,
         itemBuilder: (ctx, i) {
           if (i == 0) return _Header(detail: detail, playable: playable);
+          if (tracks.isEmpty) {
+            return Padding(
+              padding: const EdgeInsets.all(24),
+              child: Center(
+                child: Text(
+                  'No tracks yet. Add some via the "Add to playlist…" entry on any track row.',
+                  style: TextStyle(color: fs.ash),
+                  textAlign: TextAlign.center,
+                ),
+              ),
+            );
+          }
           final t = tracks[i - 1];
           return _PlaylistTrackRow(
             row: t,