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/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,
+        );
   }
 }