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

Web/src/api/icons.js

@@ -0,0 +1,41 @@
+import request from './request'
+
+/**
+ * 生成搜索关键字
+ * @param {string} categoryName - 分类名称
+ * @returns {Promise<{success: boolean, data: Array<string>>}
+ */
+export const generateSearchKeywords = (categoryName) => {
+  return request({
+    url: '/icons/search-keywords',
+    method: 'post',
+    data: { categoryName }
+  })
+}
+
+/**
+ * 搜索图标
+ * @param {Array<string>} keywords - 搜索关键字数组
+ * @returns {Promise<{success: boolean, data: Array<object>>}
+ */
+export const searchIcons = (keywords) => {
+  return request({
+    url: '/icons/search',
+    method: 'post',
+    data: { keywords }
+  })
+}
+
+/**
+ * 更新分类图标
+ * @param {number} categoryId - 分类ID
+ * @param {string} iconIdentifier - 图标标识符
+ * @returns {Promise<{success: boolean}>}
+ */
+export const updateCategoryIcon = (categoryId, iconIdentifier) => {
+  return request({
+    url: `/icons/categories/${categoryId}/icon`,
+    method: 'put',
+    data: { iconIdentifier }
+  })
+}