feat: add sound and vibration feedback for registration actions

Integrate audioplayers, vibration, and file_picker packages to provide
audio/haptic feedback on registration success/failure. Add sound
settings UI in SettingsPage for customizing notification sounds.

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

linux/flutter/generated_plugin_registrant.cc

@@ -6,6 +6,10 @@
 
 #include "generated_plugin_registrant.h"
 
+#include <audioplayers_linux/audioplayers_linux_plugin.h>
 
 void fl_register_plugins(FlPluginRegistry* registry) {
+  g_autoptr(FlPluginRegistrar) audioplayers_linux_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "AudioplayersLinuxPlugin");
+  audioplayers_linux_plugin_register_with_registrar(audioplayers_linux_registrar);
 }