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>

web/src/lib/player/store.svelte.ts

@@ -62,15 +62,34 @@ export function registerAudioEl(el: HTMLAudioElement | null): void {
   _audioEl = el;
 }
 
-export function playQueue(tracks: TrackRef[], startIndex = 0): void {
+export function playQueue(
+  tracks: TrackRef[],
+  startIndex = 0,
+  opts: { shuffle?: boolean } = {},
+): void {
   _radioSeedId = null;  // M4c: non-radio enqueue clears the radio refresh state
-  _queue = tracks;
-  if (tracks.length === 0) {
+  if (opts.shuffle && tracks.length > 1) {
+    // Fisher-Yates over the whole list. startIndex is ignored — the
+    // caller is asking for "random play from this pool," so the first
+    // track should also be random, not the one at startIndex.
+    const arr = tracks.slice();
+    for (let j = arr.length - 1; j > 0; j--) {
+      const r = Math.floor(_rng() * (j + 1));
+      [arr[j], arr[r]] = [arr[r], arr[j]];
+    }
+    _queue = arr;
     _index = 0;
-    _state = 'idle';
-  } else {
-    _index = Math.max(0, Math.min(startIndex, tracks.length - 1));
+    _shuffle = true;
     _state = 'loading';
+  } else {
+    _queue = tracks;
+    if (tracks.length === 0) {
+      _index = 0;
+      _state = 'idle';
+    } else {
+      _index = Math.max(0, Math.min(startIndex, tracks.length - 1));
+      _state = 'loading';
+    }
   }
   _position = 0;
   _duration = 0;