Add tests for upload validation, loading, and recoverable error states

[Alxandr]

Add coverage for upload UX hardening paths across validation, in-flight behavior, and recoverable failures.

Scope

• Test validation behavior in file and URL modes.
• Test loading/disabled behavior during submission.
• Test recoverable error rendering and retry behavior.

Acceptance Criteria

• Tests verify validation feedback for invalid inputs.
• Tests verify duplicate submission prevention while loading.
• Tests verify recoverable errors can be retried without reload.