feat(ui): 添加 Big.js 格式化工具，优化表格数字精度展示

src/lib/formatters.ts

@@ -0,0 +1,16 @@
+import Big from 'big.js';
+
+export function formatAmount(value: string): string {
+  return new Big(value).toFixed(2);
+}
+
+export function formatQuantity(value: string, assetType: string): string {
+  switch (assetType) {
+    case 'STOCK':
+      return new Big(value).round(0).toString();
+    case 'CRYPTO':
+      return new Big(value).round(8).toString();
+    default:
+      return new Big(value).toFixed(2);
+  }
+}