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/shared/ipc-channels.ts

@@ -92,6 +92,7 @@ export const IPC_CHANNELS = {
 
   // Logger
   LOGGER_FORWARD: 'logger:forward',
+  LOGGER_GET_LEVEL: 'logger:getLevel',
 
   // Report
   REPORT_LIST_ALL: 'report:listAll',
@@ -122,4 +123,4 @@ export const IPC_CHANNELS = {
 /**
  * Log level for logger service
  */
-export type LogLevel = 'debug' | 'info' | 'warn' | 'error'
+export type LogLevel = 'error' | 'warn' | 'info' | 'debug' | 'verbose'