Add BookPlayer scroll reading mode and floating mini-player widget

Replace paginated-only reading with a toggle between paginated (CSS columns)
and continuous vertical scroll modes. Replace the full-width fixed footer
player bar with a compact floating pill in the bottom-right corner,
expandable to show full controls, visualizer, and playlist.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

site/pages/book-player.yml

@@ -8,7 +8,9 @@ reader:
   sommaireTitle: "Sommaire"
   hints:
     desktop: "← → pages · ↑ ↓ chapitres · Esc fermer"
+    desktopScroll: "← → chapitres · Défilement libre · Esc fermer"
     mobile: "Glissez pour naviguer"
+    mobileScroll: "Défilez pour lire"
     default: "Esc pour fermer"
 pdf:
   barTitle: "Une économie du don — enfin concevable"