[Linuxuser67]

Summary

Keep potentially blocking destination-directory scans off GTK when restoring legacy queue entries or handling duplicate/missing persisted IDs.

• Precompute only the destination listings needed for fallback ID allocation, on Tokio's blocking pool.
• Pass the same snapshot to normal, gallery, and completed-history restore paths so restore does not re-read directories on GTK.
• Preserve synchronous restore wrappers for tests only.
• Propagate directory listing failures with the destination path and fail the restore without overwriting the last known-good queue; treat missing destinations as empty.
• Remove the obsolete synchronous staging-occupancy helper and use the prelisted occupancy check consistently.

Tests

• Verify only legacy or duplicate IDs require a directory listing.
• Verify fallback ID allocation skips prelisted staging names.
• Verify unreadable/non-directory destinations return an error.
• Keep the existing staging occupancy tests on the prelisted helper.

Review / verification

Reviewed the full PR diff for correctness, clarity, architecture, security, performance, and Rust best practices. This branch includes the latest main, including PR #550. GitHub CI is being run for the current head; local Cargo build/test results are not claimed.