Add API endpoints (GET/DELETE /api/performance/logs) to list and clean up
server log files by count or by age. Track the active log file path in
the logger to prevent deleting the currently open log. Add a management
UI section in the performance settings page with log directory info,
file statistics, and cleanup controls. Includes i18n translations for
all supported languages (en, fr, ja, ru, vi, zh-CN, zh-TW).
