Complete upload UX and validation hardening (file/url, loading/error/empty states)

[Alxandr]

Polish and harden the upload experience across file and URL sources with clearer validation, resilient state handling, and actionable feedback.

Scope

• Improve validation messaging for upload inputs.
• Refine loading/disabled/success/reset interactions.
• Ensure empty/error states are clear and recoverable.
• Keep behavior consistent between file and URL upload modes.

Acceptance Criteria

• Invalid inputs show actionable error messages.
• Submit/loading state prevents duplicate submissions.
• Failure states are visible, understandable, and recoverable.
• Success behavior is consistent (form reset/close and state cleanup).