feat(app.js, i18n, index.html): refactor model filtering to chart line selections

- Replaced the model filter dropdown with multiple chart line selectors for improved data visualization.
- Updated event handling to manage changes in chart line selections and refresh chart data accordingly.
- Enhanced internationalization strings for chart line labels in both English and Chinese.
- Adjusted the HTML structure to accommodate the new chart line selection UI.

i18n.js

@@ -374,8 +374,10 @@ 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.chart_line_label_1': '曲线 1',
+            'usage_stats.chart_line_label_2': '曲线 2',
+            'usage_stats.chart_line_label_3': '曲线 3',
+            'usage_stats.chart_line_hidden': '不显示',
             'usage_stats.no_data': '暂无数据',
             'usage_stats.loading_error': '加载失败',
             'usage_stats.api_endpoint': 'API端点',
@@ -877,8 +879,10 @@ 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.chart_line_label_1': 'Line 1',
+            'usage_stats.chart_line_label_2': 'Line 2',
+            'usage_stats.chart_line_label_3': 'Line 3',
+            'usage_stats.chart_line_hidden': 'Hide',
             'usage_stats.no_data': 'No Data Available',
             'usage_stats.loading_error': 'Loading Failed',
             'usage_stats.api_endpoint': 'API Endpoint',