refactor(audit): unify computerName source to cached os.hostname()

Export cachedHostname from audit-logger and use it in process-guards,
replacing process.env.COMPUTERNAME so all audit entries use the same value.

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

src/main/services/logger/audit-logger.ts

@@ -102,7 +102,7 @@ export function logAudit(
 }
 
 /** Cached hostname — invariant for the app lifecycle */
-const cachedHostname = hostname()
+export const cachedHostname = hostname()
 
 /**
  * Audit log shortcut that auto-resolves the current user context.