Client-side validation missing for custom domain setup form

[devroy10]

Description:
When setting up a custom domain in the dashboard, submitting the form with an empty domain URL field results in a raw server error (Cannot POST /api/dashboard/deployment/custom-domain/<company>) instead of a user-friendly client-side validation message.

Steps to Reproduce:

1. Go to Custom Domain Setup in the dashboard.
2. Leave the domain URL field empty.
3. Submit the form.

Expected Behavior:

• Client-side validation should prevent submission and show a friendly error (e.g., "Please enter a domain URL").
• The error should be displayed in the UI consistently with other form validations.

Actual Behavior:

• Form submits to the server with empty data.
• Raw HTML error response is displayed (Cannot POST...).

Screenshot:

Additional Context:

• Browser/OS: Chrome 137.0.7151.120 (Official Build) (64-bit) / Edge 138.0.3351.55 (Official build) (64-bit)
• This appears to be missing frontend validation that should mirror backend checks.
• The error handling seems to be generally scoped and not fine-grained.