feat: show success toast after diagram download completes

- Add optional successMessage parameter to saveDiagramToFile()
- Show toast after file download is initiated, not when dialog opens
- Add i18n strings for success message (en/ja/zh)

This is the correct implementation for issue #479 - showing feedback
after the actual save completes rather than when the save dialog opens.

components/chat-input.tsx

@@ -432,7 +432,12 @@ export function ChatInput({
                 open={showSaveDialog}
                 onOpenChange={setShowSaveDialog}
                 onSave={(filename, format) =>
-                    saveDiagramToFile(filename, format, sessionId)
+                    saveDiagramToFile(
+                        filename,
+                        format,
+                        sessionId,
+                        dict.save.savedSuccessfully,
+                    )
                 }
                 defaultFilename={`diagram-${new Date()
                     .toISOString()