feat(include): add system volume following support

Implement configurable system volume following and replace JSON IPC with binary struct.

- New Feature: Introduce `system_volume_following` CLI and runtime option.
- Enhancement: Extend CLI options, reload, and user‑config APIs to propagate the new flag.
- Enhancement: Add `VbxEvent` binary IPC struct and logging macros for better performance.
- Enhancement: Add helper to persist system volume following state.
- Bug Fix: Remove stale `parse_keyboard_event` prototype.

include/audio/playback.h

@@ -2,7 +2,6 @@
 #define VBX_AUDIO_PLAYBACK_H
 
 int init_audio(void);
-int parse_keyboard_event(const char *json_line, int *key_code, int *is_pressed);
 void play_sound_segment(int key_code, int is_pressed);
 
 #endif // VBX_AUDIO_PLAYBACK_H

include/audio/types.h

@@ -41,5 +41,7 @@ extern int g_keyboard_mute;
 extern int g_mouse_mute;
 extern int g_keyboard_enabled;
 extern int g_mouse_enabled;
+extern int g_system_volume_following;
+extern float g_system_volume_multiplier;
 
 #endif // VBX_AUDIO_TYPES_H