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/bootstrap/process-guards.ts

@@ -1,6 +1,6 @@
 import { app } from 'electron'
 import logger from '../services/logger/index'
-import { logAudit } from '../services/logger/audit-logger'
+import { logAudit, closeAuditLogger } from '../services/logger/audit-logger'
 
 export function setupProcessGuards(): void {
   process.on('uncaughtException', async (err) => {
@@ -37,4 +37,10 @@ export function setupProcessGuards(): void {
     logger.error('Child process gone', { details })
     console.error('Child process gone:', details)
   })
+
+  // Flush and close loggers before quit to prevent log loss
+  app.on('before-quit', () => {
+    logger.close()
+    closeAuditLogger()
+  })
 }