debug: add WriteDebugLog after postToIframe to confirm delivery

frontend/src/lib/CalendarPluginPage.svelte

@@ -28,9 +28,12 @@
 
   // Post message to iframe (queue if not ready)
   function postToIframe(msg) {
+    console.log('[CalendarPluginPage] postToIframe: iframeReady=' + iframeReady + ', hasEl=' + !!iframeEl + ', hasCW=' + !!(iframeEl?.contentWindow))
     if (iframeEl && iframeEl.contentWindow && iframeReady) {
+      console.log('[CalendarPluginPage] postToIframe: sending msg.type=' + msg.type)
       iframeEl.contentWindow.postMessage(msg, '*')
     } else {
+      console.log('[CalendarPluginPage] postToIframe: queuing msg.type=' + msg.type)
       messageQueue.push(msg)
     }
   }
@@ -111,6 +114,8 @@
         events,
         categories,
       })
+      // Confirm delivery via Go log
+      try { window.go.main.App.WriteDebugLog('[CalendarPluginPage] calendar-data sent to iframe, events=' + events.length + ', categories=' + (categories?.length || 0)) } catch(e) {}
     } catch (e) {
       console.error('[CalendarPluginPage] loadCalendarData:', e)
       console.log('[CalendarPluginPage] error details:', JSON.stringify({message: String(e), stack: e?.stack}))