Move shelf-read check onto the Review shelf button

[jsolly]

Summary

Moves the shelf-read completion check from beside the slice title onto the Review shelf button, and makes it smaller.

Change

• Remove the title-row green check
• Show a compact check badge on the top-right corner of Review shelf when every chapter on that shelf is marked read

Test plan

• Fully-read Basics shelf → small check on Review shelf only (14px), not beside the title
• Unread shelves still show Start learning with no check
• Mobile/desktop: badge doesn’t block taps (pointer-events-none)