refactor(logger): optimize logging architecture with 6 improvements

1. Extract shared module: consolidate getLogDir() and isProduction()
   into shared.ts, eliminate duplication across logger modules
2. Make retention config effective: delay file transport creation
   until config is loaded, apply appRetention/auditRetention from config.yaml
3. Add before-quit log flush: close logger and audit logger on
   app exit to prevent log loss
4. Unify logError entry point: remove duplicate logError from index.ts,
   re-export from error-utils.ts with richer error context
5. Renderer log level filtering: add client-side level check in preload
   to skip IPC for filtered-out messages
6. Child logger cache + audit cleanup: cache child loggers in IPC
   handler for performance, remove redundant timestamp format in audit logger

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

src/main/services/logger/error-utils.ts

@@ -6,6 +6,7 @@
  */
 
 import type { ErrorLike, SerializedError } from '../../types/errors'
+import { isProduction } from './shared'
 
 /**
  * Check if value is an Error or Error-like object
@@ -84,7 +85,7 @@ export function sanitizeError(error: SerializedError): SerializedError {
   const sanitized: SerializedError = { ...error }
 
   // Sanitize message in production
-  if (process.env.NODE_ENV === 'production') {
+  if (isProduction()) {
     // Keep error name and structure, but sanitize message
     if (sensitiveKeys.some((key) => error.message?.toLowerCase().includes(key))) {
       sanitized.message = 'An error occurred due to invalid credentials or configuration'
@@ -165,7 +166,7 @@ export function formatErrorForLogging(
   metadata: Record<string, unknown>
 } {
   const serialized = serializeError(error)
-  const isProd = process.env.NODE_ENV === 'production'
+  const isProd = isProduction()
   const errorToLog = isProd ? sanitizeError(serialized) : serialized
   const errorContext = extractErrorContext(errorToLog)