1.3.0

feat(books): store book notes as separate files

Move per-book notes out of the book JSON into their own plain-text
files under /.myne/notes/books/{id}.note, mirroring the existing
per-collection note storage. Rename the `notes` field to `note`
across the on-device catalog, web API, and dashboard.

Expose collection notes on GET /api/collections, add
SDCardManager::totalBytes() as a fallback when SD volume stats
aren't available, and simplify book-loading activities to use
BookStore::get() instead of manual JSON parsing.

Extract a shared FilePicker dashboard component and a
BooksDataProvider/useBooksData context for shared book/collection
state.

Bump firmware version to 1.3.0.