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/components/PlaylistCard.svelte

@@ -14,6 +14,11 @@
 
   const isOwn = $derived(user.value?.id === playlist.user_id);
   const isDiscover = $derived(playlist.system_variant === 'discover');
+  const isForYou = $derived(playlist.system_variant === 'for_you');
+  const isSystem = $derived(playlist.system_variant != null);
+  const refreshLabel = $derived(
+    isDiscover ? 'Refresh Discover' : isForYou ? 'Refresh For You' : 'Refresh',
+  );
 
   const queryClient = useQueryClient();
 
@@ -40,38 +45,34 @@
     if (starting) return;
     starting = true;
     try {
-      let detailID = playlist.id;
-      // For-You: refresh first so click means "give me a fresh mix
-      // and play it." Atomic replace creates a new playlist row, so
-      // the response's playlist_id is what we follow up with.
-      if (playlist.system_variant === 'for_you') {
-        const refreshed = await refreshForYou();
-        if (!refreshed.playlist_id) {
-          // Empty library — nothing to play.
-          return;
-        }
-        detailID = refreshed.playlist_id;
-        // Invalidate caches so the home + detail views see the new
-        // playlist on their next read.
-        await queryClient.invalidateQueries({ queryKey: qk.playlists() });
-      }
-      const detail = await getPlaylist(detailID);
+      const detail = await getPlaylist(playlist.id);
       const refs = toTrackRefs(detail.tracks);
       if (refs.length > 0) {
-        playQueue(refs, 0);
+        // System playlists default to shuffle so replaying within a
+        // day feels different. User playlists keep stored order.
+        // Explicit refresh moved to the kebab menu — playing no
+        // longer rebuilds the snapshot.
+        playQueue(refs, 0, { shuffle: playlist.system_variant != null });
       }
     } finally {
       starting = false;
     }
   }
 
-  async function onRefreshDiscover(e: MouseEvent) {
+  async function onRefresh(e: MouseEvent) {
     e.preventDefault();
     e.stopPropagation();
     menuOpen = false;
     try {
-      await refreshDiscover();
-      pushToast('Discover refreshed.');
+      if (isDiscover) {
+        await refreshDiscover();
+        pushToast('Discover refreshed.');
+      } else if (isForYou) {
+        await refreshForYou();
+        pushToast('For You refreshed.');
+      } else {
+        return;
+      }
       await queryClient.invalidateQueries({ queryKey: qk.playlist(playlist.id) });
       await queryClient.invalidateQueries({ queryKey: qk.playlists() });
     } catch (err: unknown) {
@@ -83,7 +84,7 @@
 <svelte:window onclick={() => { menuOpen = false; }} />
 
 <div class="card relative">
-  {#if isDiscover}
+  {#if isSystem}
     <div class="kebab-wrap absolute right-1 top-1 z-10">
       <button
         type="button"
@@ -108,11 +109,11 @@
           <button
             type="button"
             role="menuitem"
-            onclick={onRefreshDiscover}
+            onclick={onRefresh}
             class="flex w-full items-center gap-2 rounded px-2 py-1.5 text-sm text-text-primary hover:bg-surface-hover"
           >
             <RefreshCcw size={14} strokeWidth={1} />
-            Refresh Discover
+            {refreshLabel}
           </button>
         </div>
       {/if}