New cover-generator.ts module that dynamically imports foliate-js/view.js
only when cover generation is requested, keeping it out of the main bundle.
Supports all media types:
- PDF (fixed_layout): renders page 1 to canvas via view.renderer
- EPUB/CBZ (reflowable): extracts book.cover blob from parsed metadata
- Falls back to canvas-to-JPEG conversion for non-JPEG sources
