Upload progress UI and error/retry states

[ashleythedeveloper]

What

Upload progress UI and error / retry states per Figma Desktop - 19, 20.

Acceptance criteria

• Upload progress bar reflects actual bytes-sent progress against the API (not a fake animation).
• Cancel-during-upload is supported; cancels the in-flight request and resets the modal.
• Error state renders per Figma: clear error message, retry button.
• Retry resumes from the beginning (chunked / resumable upload is out of scope for v1).

Sub-issue of Epic G4.