fix: register error callback in FFI CLI binary (#575)

* fix: register error callback in FFI CLI binary

Without `on_error`, fatal errors from the background sync task were
silently swallowed and the CLI would hang waiting for Ctrl+C.

* fix: terminate process on fatal error in `on_error` callback

Addresses CodeRabbit review comment on PR #575
#575 (comment)

Author: xdustinface

dash-spv-ffi/src/bin/ffi_cli.rs

@@ -257,6 +257,23 @@ extern "C" fn on_progress_update(progress: *const FFISyncProgress, _user_data: *
     println!();
 }
 
+// ============================================================================
+// Error Callback
+// ============================================================================
+
+extern "C" fn on_error(error: *const c_char, _user_data: *mut c_void) {
+    let msg = if error.is_null() {
+        "unknown error".to_string()
+    } else {
+        unsafe { std::ffi::CStr::from_ptr(error) }
+            .to_str()
+            .unwrap_or("invalid error string")
+            .to_string()
+    };
+    eprintln!("[FATAL] {}", msg);
+    std::process::exit(1);
+}
+
 fn main() {
     let matches = Command::new("dash-spv-ffi")
         .about("Run SPV sync via FFI using event callbacks")
@@ -423,7 +440,7 @@ fn main() {
                 user_data: ptr::null_mut(),
             },
             error: FFIClientErrorCallback {
-                on_error: None,
+                on_error: Some(on_error),
                 user_data: ptr::null_mut(),
             },
         };