Two leaks from 705b171:
1. Lazy-load thumb for image-mime primary attachments created
URL.createObjectURL but never revoked. Now tracked in a
module-level registry, revoked on teardown.
2. 🔍 preview toggle's object URL same issue. Now tracked, revoked
on teardown + on toggle-off (when user clicks the preview button
to collapse).
Download button's URL (already self-cleaning via setTimeout) left
untracked — no change.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>