fix(sync): defer dataless iCloud covers

[yicheng47]

Fixes #298

Summary:

• detect modern macOS iCloud dataless cover files with a metadata stat and request download
• persist materialized covers incrementally so completed work survives interruption
• retain legacy placeholder handling and log deferred cover counts

Test plan:

• cargo test --target-dir /tmp/quill-298-target
• cargo clippy --target-dir /tmp/quill-298-target -- -D warnings
• git diff --check