Resume after refresh

Finish the transfer flow. Read RELIABILITY.md before transfer code. Optional tooling and UI follow-up in 02-pre-upload-readiness.md does not block this milestone.

Remaining work:

• Pass the 10 GB internal gate, then the 100 GB and 350 GB gates. Record correctness, avoidable bytes resent and manual intervention in the PR or issue.
• Restore persistent FileSystemFileHandles where Chromium supports them, so a refresh needs no reselection.
• Cross-tab ownership (e.g. Web Locks), so a second tab doesn't offer to resume an upload another tab is running.
• Upstream the Uppy ListParts pagination fix and drop the patch.

Run existing tests and direct runtime checks. New test files still require explicit approval.