Poor performance when CloudKit is doing stuff

[katagaki]

When CloudKit is syncing, illustrations grid reloads every few seconds, causing a really bad freeze/lag when scrolling.

[katagaki]

Store images and thumbnails in filesystem and not SwiftData/CloudKit:

• [x] Implement a directory structure for storage.
• [x] - One folder for actual image files
• [x] - One folder for thumbnails
• [x] Reliably link files from the filesystem (Documents folder in Files app) to the app (SwiftData).
• [x] - Need to consider that albums can have covers
• [ ] Implement a way to easily import files directly into the folder.
• [ ] - Files imported will go directly into the root album
• [ ] - Imported files automatically moved and linked from import folder into the folder for actual image files
• [x] Implement iCloud syncing for folder.
• [ ] Implement notice for iCloud sync being required

[katagaki]

All other enhancements will be tracked in separate issues.