iOS: add floating audio button when player is not active

- Moved audio start button from top-right toolbar to floating bottom-right position
- Only shows floating button when audio player is not active
- Button styled as amber pill with 'Listen' text and play icon
- More accessible placement for quick audio playback start
- Positioned 20pt from bottom and right edges with shadow

ios/LibNovel/LibNovel/Views/ChapterReader/ChapterReaderView.swift

@@ -48,7 +48,12 @@ struct ChapterReaderView: View {
         }
         .navigationTitle(vm.content.map { "Ch.\($0.chapter.number)" } ?? "")
         .navigationBarTitleDisplayMode(.inline)
-        .toolbar { audioToolbarButton }
+        .overlay(alignment: .bottomTrailing) {
+            // Floating audio button when player is not active
+            if !audioPlayer.isActive {
+                floatingAudioButton
+            }
+        }
         .task {
             print("[ChapterReaderView] .task fired — calling vm.load()")
             await vm.load()
@@ -140,7 +145,34 @@ struct ChapterReaderView: View {
         }
     }
 
-    // MARK: - Audio toolbar button
+    // MARK: - Floating audio button
+
+    @ViewBuilder
+    private var floatingAudioButton: some View {
+        Button {
+            vm.toggleAudio(audioPlayer: audioPlayer, settings: authStore.settings)
+        } label: {
+            HStack(spacing: 8) {
+                Image(systemName: "play.circle.fill")
+                    .font(.system(size: 22))
+                Text("Listen")
+                    .font(.subheadline.weight(.semibold))
+            }
+            .foregroundStyle(.white)
+            .padding(.horizontal, 20)
+            .padding(.vertical, 12)
+            .background(
+                Capsule()
+                    .fill(Color.amber)
+                    .shadow(color: .black.opacity(0.25), radius: 8, y: 4)
+            )
+        }
+        .buttonStyle(.plain)
+        .padding(.trailing, 20)
+        .padding(.bottom, 20)
+    }
+
+    // MARK: - Audio toolbar button (kept for when player is active)
 
     @ToolbarContentBuilder
     private var audioToolbarButton: some ToolbarContent {