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>

app/components/layout/TheFooter.vue

@@ -1,5 +1,5 @@
 <template>
-  <footer class="border-t border-white/8 bg-surface-600 pb-[var(--player-height)]">
+  <footer class="border-t border-white/8 bg-surface-600">
     <div class="container-content px-4 py-8">
       <div class="flex flex-col items-center gap-4 md:flex-row md:justify-between">
         <!-- Credits -->