first commit

db.js

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+const DB_NAME = 'dww11-account-switch-v3';
+let opening;
+export function openDB() {
+  if (!opening) opening = new Promise((resolve, reject) => {
+    const req = indexedDB.open(DB_NAME, 1);
+    req.onupgradeneeded = () => {
+      req.result.createObjectStore('accounts', { keyPath: 'username' });
+      req.result.createObjectStore('meta');
+    };
+    req.onsuccess = () => {
+      const db = req.result;
+      db.onversionchange = () => { db.close(); opening = null; };
+      db.onclose = () => { opening = null; };
+      resolve(db);
+    };
+    req.onerror = () => { opening = null; reject(new Error('无法打开 IndexedDB：' + req.error?.message)); };
+    req.onblocked = () => { opening = null; reject(new Error('数据库升级被阻塞，请关闭扩展管理页后重试。')); };
+  });
+  return opening;
+}
+
+// 所有状态变更跨 accounts/meta 使用同一事务。回调必须同步，避免 await 使事务自动提交。
+export async function transact(reducer, write = true) {
+  const db = await openDB();
+  return new Promise((resolve, reject) => {
+    const tx = db.transaction(['accounts', 'meta'], write ? 'readwrite' : 'readonly', write ? { durability: 'strict' } : undefined);
+    const accountsStore = tx.objectStore('accounts'), metaStore = tx.objectStore('meta');
+    const a = accountsStore.getAll(), m = metaStore.get('state');
+    let output, failure, ready = 0;
+    const loaded = () => {
+      if (++ready !== 2) return;
+      try {
+        const state = m.result || { schedule: null, flow: null };
+        output = reducer(a.result, state, accountsStore);
+        if (write) metaStore.put(state, 'state');
+      } catch (e) { failure = e; tx.abort(); }
+    };
+    a.onsuccess = loaded; m.onsuccess = loaded;
+    tx.oncomplete = () => resolve(output);
+    tx.onabort = () => reject(failure || new Error('数据库事务未提交：' + (tx.error?.message || '已中止')));
+    tx.onerror = () => {}; // 由 onabort 统一处理，不在提交之前返回成功。
+  });
+}