test: 添加核心服务回归测试

- 为存储、词库、记忆调度、MIME 解析、单词提取、测验生成及备份处理添加 Node.js 测试
- 添加可复用的浏览器环境与 localStorage 测试辅助工具
- 修复移除 HTML 时单词边界丢失的问题，并拒绝无效的测验答案
- 校验导入备份中的日期，并添加 npm 测试脚本

src/js/pages/settings-data.js

@@ -99,6 +99,13 @@ export function handleImportBackup(input) {
   reader.readAsText(file);
 }
 
+function isValidBackupDate(value) {
+  if (typeof value !== 'string' || !/^\d{4}-\d{2}-\d{2}$/.test(value)) return false;
+  const [year, month, day] = value.split('-').map(Number);
+  const date = new Date(year, month - 1, day);
+  return date.getFullYear() === year && date.getMonth() === month - 1 && date.getDate() === day;
+}
+
 function normalizeBackupCategory(value) {
   return typeof value === 'string' ? value.trim().slice(0, 100) : '';
 }
@@ -153,9 +160,7 @@ function normalizeBackupSchedule(value) {
       const numericCount = Number(count);
       return Number.isFinite(numericCount) ? Math.max(0, Math.trunc(numericCount)) : 0;
     };
-    const nextReview = typeof raw.nextReview === 'string' && /^\d{4}-\d{2}-\d{2}$/.test(raw.nextReview)
-      ? raw.nextReview
-      : getToday();
+    const nextReview = isValidBackupDate(raw.nextReview) ? raw.nextReview : getToday();
     schedule[numericWordId] = {
       stage,
       nextReview,
@@ -192,7 +197,7 @@ function normalizeBackupRecords(value) {
     return {
       ...raw,
       wordId,
-      date: typeof raw.date === 'string' && /^\d{4}-\d{2}-\d{2}$/.test(raw.date) ? raw.date : getToday(),
+      date: isValidBackupDate(raw.date) ? raw.date : getToday(),
       time: Number.isFinite(time) && time >= 0 ? time : Date.now(),
       isCorrect: raw.isCorrect,
       type: raw.type,