Refresh notebook themes off the frame thread, retain the last good cache on read failures, and apply loaded themes when the worker reports back. Preserve newer local theme edits when a slower read completes.

Verified with a stalled FIFO theme read: the previous release stops painting; the fixed app opens Options and switches appearance before the read completes. App tests, Clippy, and formatting pass.

Assisted-by: gpt-6.1-sol