chore: migrate remaining ECharts components to Chart.js

- Migrated 4 components from ECharts to Chart.js:
  * MonthlyExpenseCard.vue (折线图)
  * DailyTrendChart.vue (双系列折线图)
  * ExpenseCategoryCard.vue (环形图)
  * BudgetChartAnalysis.vue (仪表盘 + 多种图表)

- Removed all ECharts imports and environment variable switches
- Unified all charts to use BaseChart.vue component
- Build verified: pnpm build success ✓
- No echarts imports remaining ✓

Refs: openspec/changes/migrate-remaining-echarts-to-chartjs

Service/IconSearch/IIconSearchService.cs

@@ -0,0 +1,29 @@
+namespace Service.IconSearch;
+
+/// <summary>
+/// 图标搜索服务接口
+/// </summary>
+public interface IIconSearchService
+{
+    /// <summary>
+    /// 生成搜索关键字
+    /// </summary>
+    /// <param name="categoryName">分类名称</param>
+    /// <returns>搜索关键字数组</returns>
+    Task<List<string>> GenerateSearchKeywordsAsync(string categoryName);
+
+    /// <summary>
+    /// 搜索图标并返回候选列表
+    /// </summary>
+    /// <param name="keywords">搜索关键字数组</param>
+    /// <param name="limit">每个关键字返回的最大图标数量</param>
+    /// <returns>图标候选列表</returns>
+    Task<List<IconCandidate>> SearchIconsAsync(List<string> keywords, int limit = 20);
+
+    /// <summary>
+    /// 更新分类图标
+    /// </summary>
+    /// <param name="categoryId">分类ID</param>
+    /// <param name="iconIdentifier">图标标识符</param>
+    Task UpdateCategoryIconAsync(long categoryId, string iconIdentifier);
+}