feat(server): Report restore task progress - #5572

Summary

• report item and byte progress for restores started through the server API
• include completion percentage, transfer speed, estimated remaining time, skipped data, and ignored errors
• register individual file downloads as cancellable Restore tasks
• handle byte-based, zero-byte, skipped, and root-entry progress consistently
• document restore progress in the KopiaUI getting-started guide
• add server tests for progress calculation and streamed object downloads

Why

Restore operations in KopiaUI previously provided no useful indication of how much work had completed. Individual file downloads also bypassed the restore endpoint entirely, so they did not appear on the Tasks page.

This change uses the restore statistics already produced by Kopia's restore implementation and reports them through the existing task controller. Individual downloads are streamed synchronously as before while also being represented by a Restore task.

Closes #3609.

Companion HTML UI PR: kopia/htmlui#468

Validation

• go test ./internal/server ./snapshot/restore
• git diff --check
• end-to-end testing in a locally built macOS ARM64 KopiaUI application
• verified directory restores and individual file downloads appear in Tasks with an incrementing percentage