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/preload/api/logger.ts

@@ -2,13 +2,42 @@ import type { LogLevel } from '../../shared/ipc-channels'
 import { IPC_CHANNELS } from '../../shared/ipc-channels'
 import { ipcRenderer } from '../lib/ipc'
 
+// Cached log level for client-side filtering (avoids IPC for filtered-out messages)
+let cachedLevel: LogLevel = 'info'
+
+/**
+ * Check if a message at the given level should be logged
+ * Based on level priority: error > warn > info > debug > verbose
+ */
+function shouldLog(level: LogLevel): boolean {
+  const priorities: Record<LogLevel, number> = {
+    verbose: 0,
+    debug: 1,
+    info: 2,
+    warn: 3,
+    error: 4
+  }
+  return (priorities[level] ?? 0) >= (priorities[cachedLevel] ?? 2)
+}
+
 export const loggerApi = {
   log: (level: LogLevel, message: string, context?: Record<string, unknown>): void => {
+    // Drop messages below the configured log level
+    if (!shouldLog(level)) return
+
     ipcRenderer.send(IPC_CHANNELS.LOGGER_FORWARD, {
       level,
       message,
       context,
       timestamp: Date.now()
     })
+  },
+
+  /**
+   * Fetch the current log level from main process and cache it
+   * Should be called early in renderer initialization
+   */
+  fetchLevel: async (): Promise<void> => {
+    cachedLevel = (await ipcRenderer.invoke(IPC_CHANNELS.LOGGER_GET_LEVEL)) as LogLevel
   }
 } as const

src/preload/index.d.ts

@@ -129,6 +129,7 @@ export interface ConfigAPI {
 
 export interface LoggerAPI {
   log: (level: LogLevel, message: string, context?: Record<string, unknown>) => void
+  fetchLevel: () => Promise<void>
 }
 
 export interface UpdateAPI {