debug: add visible SFTP diagnostic banner + WS error/close handlers

frontend/components/sftp/SftpSidebar.vue

@@ -103,6 +103,10 @@ const breadcrumbs = computed(() => {
 <template>
   <div class="flex h-full" :style="{ width: width + 'px' }">
     <div class="flex flex-col h-full bg-gray-900 border-r border-gray-800 overflow-hidden flex-1">
+      <!-- Debug banner — remove once SFTP is working -->
+      <div class="px-2 py-1 bg-yellow-900/50 text-yellow-300 text-xs border-b border-yellow-800">
+        SFTP: {{ entries.length }} entries | path: {{ currentPath }} | sid: {{ sessionId?.substring(0, 8) }}
+      </div>
       <!-- Toolbar -->
       <div class="px-2 py-1.5 border-b border-gray-800 shrink-0">
         <!-- Breadcrumbs -->

frontend/composables/useSftp.ts

@@ -59,6 +59,14 @@ export function useSftp(sessionId: Ref<string | null>) {
       }
     }
 
+    ws.onerror = (event) => {
+      console.error('[SFTP] WS error event:', event)
+    }
+
+    ws.onclose = (event) => {
+      console.log('[SFTP] WS closed, code:', event.code, 'reason:', event.reason, 'wasClean:', event.wasClean)
+    }
+
     return ws
   }