Add invoice validation before save to prevent empty/incomplete invoices

[Prachi01Yadav]

Problem

Currently there is zero validation before an invoice is saved -
locally or to Firebase. A user can save a completely empty invoice
with no name, no vendor, no amount. This causes silent data quality
issues and corrupted invoice records.

Proposed Fix

Add a client-side validation service that runs before any save
operation and checks:

• Invoice has a meaningful name (not empty or "default")
• Invoice content is not empty
• Warns if key fields appear unfilled

Files to change

• New file: src/services/InvoiceValidator.ts
• Edit: src/components/Menu/Menu.tsx (call validator before save)

Related

Part of the Co-Pilot validation work in Issue #37