fix(reader): add floating back button to exit pdf viewer

The PDF reader lacked an exit button, trapping users. Since the native PDF viewer header is packed with tools and difficult to safely inject into, a floating action button (FAB) is added to the bottom left of the screen.

Closes #290

client/src/features/reader/pdf-v4/PdfV4ReaderView.vue

@@ -10,6 +10,7 @@ import { useReaderSettings } from '../shared/composables/useReaderSettings'
 import { useThemeStore, ACCENT_OPTIONS } from '@/stores/theme'
 import type { PdfReaderSettings } from '@bookorbit/types'
 import { getIsDark, lookupAccentHex } from './pdf-viewer-utils'
+import { ArrowLeft } from 'lucide-vue-next'
 
 const props = defineProps<{ bookId: number; fileId: number; peekMode?: boolean }>()
 const route = useRoute()
@@ -189,6 +190,17 @@ onUnmounted(() => {
 
 <template>
   <div class="fixed inset-0 flex flex-col">
+    <!-- Floating Back Button (FAB) -->
+    <div class="pointer-events-none absolute bottom-6 left-6 z-50 flex">
+      <button
+        class="pointer-events-auto flex h-12 w-12 items-center justify-center rounded-full bg-background/95 text-foreground shadow-lg border border-border backdrop-blur-md transition-transform hover:scale-105 active:scale-95"
+        @click="router.back()"
+        aria-label="Go back"
+      >
+        <ArrowLeft :size="22" />
+      </button>
+    </div>
+
     <div v-if="props.peekMode" class="pointer-events-none absolute left-0 right-0 top-3 z-50 flex justify-center">
       <div class="pointer-events-auto flex h-8 items-center gap-2 rounded-md border border-primary/30 bg-background/95 px-2 text-primary shadow-sm">
         <span class="text-xs font-medium">Peeking</span>