Set preferred timezone in Configuration

[faangbait]

Those of us who work inside sandboxes/devcontainers often don't have a timezone matching our actual timezone. Allow the selection of a timezone inside the Preferences panel. e.g.

  private getConfiguration(): ExtensionConfig {
    const config = vscode.workspace.getConfiguration('claudeCodeUsage');
    return {
      refreshInterval: config.get('refreshInterval', 60),
      dataDirectory: config.get('dataDirectory', ''),
      language: config.get('language', 'auto'),
      decimalPlaces: config.get('decimalPlaces', 2),
      timezone: config.get('timezone', 'en-US')             // <---- new
    };
  }