feat(app.js, i18n, index.html, styles.css): implement model filtering in usage statistics

- Added a model filter dropdown to the usage statistics UI, allowing users to filter data by model.
- Implemented methods to handle model filter changes and update chart data accordingly.
- Enhanced internationalization strings for model filter labels in both English and Chinese.
- Updated styles for the model filter to improve layout and user experience.

i18n.js

@@ -374,6 +374,8 @@ const i18n = {
             'usage_stats.by_hour': '按小时',
             'usage_stats.by_day': '按天',
             'usage_stats.refresh': '刷新',
+            'usage_stats.model_filter_label': '模型筛选',
+            'usage_stats.model_filter_all': '所有模型',
             'usage_stats.no_data': '暂无数据',
             'usage_stats.loading_error': '加载失败',
             'usage_stats.api_endpoint': 'API端点',
@@ -875,6 +877,8 @@ const i18n = {
             'usage_stats.by_hour': 'By Hour',
             'usage_stats.by_day': 'By Day',
             'usage_stats.refresh': 'Refresh',
+            'usage_stats.model_filter_label': 'Model Filter',
+            'usage_stats.model_filter_all': 'All Models',
             'usage_stats.no_data': 'No Data Available',
             'usage_stats.loading_error': 'Loading Failed',
             'usage_stats.api_endpoint': 'API Endpoint',