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.

index.html

@@ -825,6 +825,16 @@
                                 </div>
                             </div>
 
+                            <!-- 模型筛选 -->
+                            <div class="usage-filter-bar">
+                                <div class="usage-filter-group">
+                                    <label for="model-filter-select" data-i18n="usage_stats.model_filter_label">模型筛选</label>
+                                    <select id="model-filter-select" class="model-filter-select" disabled>
+                                        <option value="all" data-i18n="usage_stats.model_filter_all">所有模型</option>
+                                    </select>
+                                </div>
+                            </div>
+
                             <!-- 图表区域 -->
                             <div class="charts-container">
                                 <!-- 请求趋势图 -->