fix: add validation to prevent saving empty or incomplete invoices

[Mikey3600]

Summary

Fixes #48

Adds validation to prevent users from saving empty or incomplete invoices in both Save and Save As workflows.

Changes

• Added empty content check in doSave() before saving — shows toast message if invoice is empty
• Added empty content check in doSaveAs() before saving — same validation applied consistently
• User sees "Cannot save an empty invoice!" toast message when attempting to save empty spreadsheet
• Prevents corrupt or blank invoice records from being stored

Testing

• Tested Save with empty spreadsheet — toast message appears correctly
• Tested Save As with empty spreadsheet — toast message appears correctly
• Tested Save with valid content — saves successfully as before
• No regressions in existing save functionality

PR Category

• Bug Fix