feat(playlists): system playlists default to shuffle on tile play

Closes Fable #412 (For You force-refresh on play) and #413
(shuffle-on-play default for system playlists).

Web (PlaylistCard.svelte + player/store.svelte.ts):
- Drop the refreshForYou() call from the play handler. The daily
  03:00 user-local snapshot is what plays now. Stops burning server
  compute on every press and stops swapping the playlist out from
  under the user.
- Generalize the kebab affordance to render for any system playlist
  (was Discover-only). Adds "Refresh For You" as an explicit
  replacement so users can still force a regen when they want one.
- Extend playQueue(tracks, startIndex, { shuffle? }) to Fisher-Yates
  the queue when shuffle:true. PlaylistCard passes shuffle:true for
  any non-null system_variant.

Flutter (player_provider.dart + playlists/widgets/playlist_card.dart):
- playTracks now accepts shuffle:bool. When true, picks a random
  starting index and enables AudioServiceShuffleMode.all after
  setQueueFromTracks. PlaylistCard passes shuffle:playlist.isSystem.

User playlists keep linear order. Detail-screen play buttons are
unchanged for now (follow-up if user requests).

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

flutter_client/lib/player/player_provider.dart

@@ -1,3 +1,5 @@
+import 'dart:math' show Random;
+
 import 'package:audio_service/audio_service.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
@@ -65,7 +67,18 @@ class PlayerActions {
   }
   final Ref _ref;
 
-  Future<void> playTracks(List<TrackRef> tracks, {int initialIndex = 0}) async {
+  Future<void> playTracks(
+    List<TrackRef> tracks, {
+    int initialIndex = 0,
+    bool shuffle = false,
+  }) async {
+    // shuffle=true means "play this pool randomly, starting at a
+    // random track." Used for system playlists so the first track of
+    // a re-play within the day is different from the prior one.
+    var startAt = initialIndex;
+    if (shuffle && tracks.length > 1) {
+      startAt = Random().nextInt(tracks.length);
+    }
     final url   = await _ref.read(serverUrlProvider.future);
     final token = await _ref.read(secureStorageProvider).read(key: 'session_token');
     final cache = _ref.read(albumCoverCacheProvider);
@@ -78,7 +91,10 @@ class PlayerActions {
         audioCacheManager: audioCache,
         likeBridge: _buildLikeBridge(),
       );
-    await h.setQueueFromTracks(tracks, initialIndex: initialIndex);
+    await h.setQueueFromTracks(tracks, initialIndex: startAt);
+    if (shuffle) {
+      await h.setShuffleMode(AudioServiceShuffleMode.all);
+    }
     await h.play();
   }
 

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -120,6 +120,13 @@ class PlaylistCard extends ConsumerWidget {
       ));
     }
     if (refs.isEmpty) return;
-    await ref.read(playerActionsProvider).playTracks(refs, initialIndex: 0);
+    // System playlists default to shuffle so replaying within a day
+    // doesn't deliver the identical experience. User playlists keep
+    // stored order.
+    await ref.read(playerActionsProvider).playTracks(
+          refs,
+          initialIndex: 0,
+          shuffle: playlist.isSystem,
+        );
   }
 }