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/app/cli.h

@@ -15,6 +15,7 @@ typedef struct {
   int mouse_mute;    // -1 unchanged, 0 unmute, 1 mute
   int keyboard_enabled; // -1 unchanged, 0 disabled, 1 enabled
   int mouse_enabled;    // -1 unchanged, 0 disabled, 1 enabled
+  int system_volume_following; // -1 unchanged, 0 disabled, 1 enabled
 } CliOptions;
 
 void print_usage(const char *program_name);