fix(ui): initialize DocumentImportSystem so file attachments work

documentImportSystem was declared as @State but never initialized,
so the guard in sendMessage() always failed and silently discarded
attached files. Now initialized in performMainChatViewAppear().

Author: fewtarius

Sources/UserInterface/Chat/ChatWidget.swift

@@ -2401,6 +2401,11 @@ public struct ChatWidget: View {
         // Initialize ModelListManager with dependencies
         modelListManager.initialize(endpointManager: endpointManager)
 
+        // Initialize DocumentImportSystem for file attachments
+        if documentImportSystem == nil {
+            documentImportSystem = DocumentImportSystem(conversationManager: conversationManager)
+        }
+        
         // ModelListManager handles model loading automatically
         loadSystemPrompts()
         loadRecentChatSession()