Harden empty and error state rendering in upload dialog

[Alxandr]

Ensure upload surfaces handle empty data and error scenarios cleanly without broken UI states.

Scope

• Handle empty tag/source-dependent states in upload dialog.
• Render resilient fallback UI for load and mutation errors.
• Ensure dialog remains usable after recoverable failures.

Acceptance Criteria

• Empty states are intentional and understandable.
• Error states do not leave dialog in a broken state.
• Users can recover and retry after recoverable errors.