fix(player-web): don't optimistic-render fullscreen when layout is unknown

Follow-up to the layout cache. On a cold start with a cached playlist but no cached
layout yet (first run after shipping, or cleared cache), the player still rendered
fullscreen and flashed before the payload arrived. Now gate the optimistic cached
render on the layout being KNOWN (cache key present — null=fullscreen vs object=
zoned, both fine); if unknown, wait ~1s for the payload to drive the first render.
Eliminates the fullscreen flash on the very first pass too.

server/player/index.html

@@ -517,13 +517,18 @@
     }
 
     if (config.serverUrl && config.deviceId && config.paired) {
-      // Restore cached playlist immediately so content plays even if offline
+      // Restore cached playlist immediately so content plays even if offline —
+      // but ONLY if we also know the layout, else we'd guess fullscreen and flash
+      // before the payload arrives. Key presence (not value) is the test: an absent
+      // key means "layout unknown" (e.g. first run after this shipped, or cleared
+      // cache), while a stored `null` is a real fullscreen device. Both caches are
+      // written on every payload, so after the first connection this always renders
+      // immediately; only a genuinely-unknown layout waits (~1s) for the payload.
       const cachedPlaylist = loadPlaylistCache();
-      if (cachedPlaylist.length > 0) {
+      const layoutKnown = localStorage.getItem(LAYOUT_CACHE_KEY) !== null;
+      if (cachedPlaylist.length > 0 && layoutKnown) {
         console.log('Restored cached playlist:', cachedPlaylist.length, 'items');
         playlist = cachedPlaylist;
-        // Restore the cached layout too, so the first render is already zoned
-        // (otherwise the cold start shows fullscreen until the payload arrives).
         layout = loadLayoutCache();
         currentIndex = 0;
         isPlaying = true;