File Vault Private & Encrypted

- Added a dedicated heavy-thumbnail image generator script for stress-testing add-page thumbnail
loading with 50 large images across multiple folders.
- Reworked add-page media scanning to use in-memory caching, parallel subtree search, and
progressive async result rendering instead of one blocking storage walk.
- Updated media picker presentation to sort by folder first, then by date within each folder, with
only one folder section expanded at a time.