feat logging: Implement LOG_LEVEL configuration option and improve logging consistency

- Add LOG_LEVEL configuration option to control logging verbosity
- Update UI to include LOG_LEVEL dropdown in settings
- Fix logging inconsistency between environment variables and config file
- Unify logging configuration to use config file settings
- Maintain separate logging systems for different purposes:
  * Server-level logs (HTTP requests, API calls) using pino in ~/.claude-code-router/logs/
  * Application-level logs (routing decisions, business logic) in ~/.claude-code-router/claude-code-router.log
- Update documentation with accurate logging system information
- Add detailed information about dual logging systems in README.md and README_zh.md
- Improve type safety and validation in ConfigProvider

Co-authored-by: qwen-cli <https://github.com/QwenLM/qwen-code>

ui/src/locales/en.json

@@ -36,6 +36,7 @@
   "toplevel": {
     "title": "General Settings",
     "log": "Enable Logging",
+    "log_level": "Log Level",
     "claude_path": "Claude Path",
     "host": "Host",
     "port": "Port",