fix: refine linux voice runtime behavior

apps/chanora_flutter/lib/src/rust/api.dart

@@ -198,7 +198,8 @@ Future<void> sendChatMessage({
 /// On non-Android targets or before a voice session opens the
 /// section is omitted. Per SDD-090 every field in that section is
 /// a device-side technical scalar — no PII admitted.
-String exportDiagnostics() => RustLib.instance.api.crateApiExportDiagnostics();
+Future<String> exportDiagnostics() =>
+    RustLib.instance.api.crateApiExportDiagnostics();
 
 /// Wire the identity persistence store to a platform-private
 /// directory. Should be called once on app start after Flutter has