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/layouts/default.vue

@@ -1,7 +1,7 @@
 <template>
   <div class="app-layout grid grid-cols-1 min-h-dvh">
     <LayoutTheHeader />
-    <main class="pb-[var(--player-height)]">
+    <main>
       <slot />
     </main>
     <LayoutTheFooter />