refactor(ui): replace SVG artwork placeholders with CSS

[alexhb1]

Description

Currently, books with no artwork use a generated SVG with specific (ugly) styling and has a complex and intensive pipeline for displaying them, especially with large libraries. They're generated on the fly with no storage or caching. This replaces the entire approach with a simplified in-line placeholder div with a bit of basic text and CSS.

Before vs after:

Changes

• Removes all SVG generating code and plumbing for displaying placeholder images
• Adds a basic CSS placeholder with title+author and background color

Will mark as ready once my other layout shell rework PR is done and through, this can be sorted afterwards.

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[balazs-szucs]

Currently, books with no artwork use a generated SVG with specific (ugly) styling and has a complex and intensive pipeline for displaying them, especially with large libraries

🤣