fix(player): zone reset on multi->single layout switch + don't blank multi-zone

- Server (deviceSocket buildPlaylistPayload): when a device's layout has <2 zones
  (single or none), strip leftover zone_id from assignments. After switching a
  device from multi-zone back to fullscreen, content was stuck bound to a gone
  left/right zone_id and never played; nulling it lets both players fall back to
  the default fullscreen renderer.
- Web player: render multi-zone zones BEFORE the single-item 'renderable?' bail,
  so an empty/placeholder current rotation item can't blank the whole screen.

server/player/index.html

@@ -1320,6 +1320,14 @@
       const container = document.getElementById('playerContainer');
       container.style.display = 'block';
 
+      // Multi-zone: each zone pulls its own content by zone_id, independent of the
+      // rotating "current item". Render zones here (before the single-item bail) so
+      // an empty/placeholder current item can't blank the whole multi-zone screen.
+      if (layout && layout.zones && layout.zones.length > 1 && !wallConfig) {
+        renderZones(container, item);
+        return;
+      }
+
       // Defense in depth: bail to waiting state on missing/malformed item rather
       // than fall through every branch and leave a blank container.
       const hasRenderableType = item && (

server/ws/deviceSocket.js

@@ -154,6 +154,16 @@ function buildPlaylistPayload(deviceId) {
     }
   }
 
+  // Zone reset: if the device isn't in a real multi-zone layout (single zone or
+  // no layout), strip any leftover zone_id from assignments. Otherwise, after
+  // switching a device from a multi-zone layout back to single/fullscreen, the
+  // content stays bound to a now-gone left/right zone_id and never plays. With
+  // zone_id nulled, both players fall back to the default fullscreen renderer.
+  const zoneCount = layout?.zones?.length || 0;
+  if (zoneCount < 2 && Array.isArray(assignments)) {
+    assignments = assignments.map(a => (a && a.zone_id != null ? { ...a, zone_id: null } : a));
+  }
+
   return { assignments, layout, orientation: device?.orientation || 'landscape', wall_config };
 }