fix(logger): address code review findings

- Remove misleading await from audit-logger tests (functions are sync)
- Add cleanup() to LoggerAPI type definition in index.d.ts
- Fix circular reference fallback to preserve null values

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

src/preload/index.d.ts

@@ -130,6 +130,7 @@ export interface ConfigAPI {
 export interface LoggerAPI {
   log: (level: LogLevel, message: string, context?: Record<string, unknown>) => void
   fetchLevel: () => Promise<void>
+  cleanup: () => void
 }
 
 export interface UpdateAPI {