[BUG]: Negative invoice line amount shown and invalid invoice submission allowed

[Atharva0506]

Bug Description

When creating an invoice, entering values where Discount is greater than Qty × Unit Price can produce a negative line amount in the UI. In this state, invoice creation behavior is inconsistent and users do not get clear validation feedback.

This should be prevented with proper validation and standardized toast feedback.

Steps to Reproduce

1. Go to Dashboard → Create Invoice
2. Add an item:
   • Description: Test
   • Qty: 10
   • Unit Price: 10
   • Discount: 1000
   • Tax: 0
3. Observe the line amount display
4. Try clicking Create Invoice

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Expected Behavior

• Negative amounts should never be accepted.
• Users should see a clear toast/form validation message explaining what is wrong.
• Invoice creation should be blocked until values are valid.
• Total amount due should remain consistent with line-item validation.

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Actual Behavior

• UI can show a negative line amount in this scenario.
• Validation feedback is unclear/inconsistent for invalid negative amount states.
• Invalid data may proceed further than expected in the create flow.

Logs and Screenshots

Environment Details

• OS: Windows 11
• Browser: Chrome
• Network: Any supported chain
• Wallet: Metamask

Impact

High - Major feature is broken

Code of Conduct

• I have joined the Discord server and will post updates there
• I have searched existing issues to avoid duplicates