Improve field-level validation and actionable submit errors

[Alxandr]

Strengthen upload validation and error feedback so users can quickly resolve invalid input.

Scope

• Improve field-level validation feedback for required/invalid values.
• Normalize and validate URL/file mode inputs consistently.
• Provide actionable submit-time errors for server-side failures.

Acceptance Criteria

• Field-level errors identify the specific invalid input.
• Submit errors communicate what failed and what to fix.
• Validation behavior is consistent across file and URL modes.