Show success toast after saving diagram (#484)

* Add success toast after saving diagram

* fix: correct save toast placement

* Changes made:
1. Added i18n support
2. Fixed the issue where the save toast was running only once

* fix: show toast after download completes, not when dialog opens

Move toast from handleDrawioSave (dialog open) to saveDiagramToFile
(after download). Also restore the duplicate-save guard that was removed.

---------

Co-authored-by: Biki Kalita <86558912+Biki-dev@users.noreply.github.com>
Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

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()