fix: synchronize webapp container state access

app/src/main/java/top/yeij/hearth/webapp/WebAppContainer.kt

@@ -9,12 +9,21 @@ data class Tab(val id: String, val url: String, val active: Boolean = false, val
 // 真实 WebView 导航（goBack/goForward/reload）留待 Task 13 集成时接入
 // Multi-tab state management (pure Kotlin data layer, WebView-free, JVM-testable)
 // Real WebView navigation (goBack/goForward/reload) is wired up in Task 13
+//
+// 状态方法全部 @Synchronized：open/close/switchTo 写操作在主线程，listTabs/tabs 读操作
+// 在 JavaBridge 线程，跨线程读写同一 mutableListOf，需用同一把锁串行，避免
+// ConcurrentModificationException（尤其 tabs() 的 toList() 迭代与写操作互斥）
+// All state methods are @Synchronized: open/close/switchTo write on the main thread
+// while listTabs/tabs read on the JavaBridge thread; reading and writing the same
+// mutableListOf across threads must serialize on a single lock to avoid
+// ConcurrentModificationException (especially toList() iteration vs writes)
 class WebAppContainer {
     private val tabs = mutableListOf<Tab>()
 
     // 打开标签：已存在则激活它（不重复添加），否则新增并激活；name 记录展示名
     // Open a tab: if it already exists just activate it, otherwise add and activate;
     // name records the display title from the manifest
+    @Synchronized
     fun open(id: String, url: String, name: String = "") {
         val existing = tabs.find { it.id == id }
         if (existing != null) {
@@ -27,6 +36,7 @@ class WebAppContainer {
 
     // 关闭标签：若关闭后没有激活标签则激活第一个
     // Close a tab: if none remain active, activate the first one
+    @Synchronized
     fun close(id: String) {
         tabs.removeAll { it.id == id }
         if (tabs.isNotEmpty() && tabs.none { it.active }) {
@@ -36,6 +46,7 @@ class WebAppContainer {
 
     // 切换激活标签：id 不存在时返回 false 且不改动状态（保持原激活标签）
     // Switch the active tab: return false and keep current state when id is unknown
+    @Synchronized
     fun switchTo(id: String): Boolean {
         if (tabs.none { it.id == id }) return false
         setActive(id)
@@ -44,10 +55,12 @@ class WebAppContainer {
 
     // 返回标签列表副本（外部无法直接改动内部状态）
     // Return a defensive copy of the tab list
+    @Synchronized
     fun tabs(): List<Tab> = tabs.toList()
 
     // 返回当前激活标签 id，无标签或未激活返回 null
     // Return the active tab id, or null when empty/none active
+    @Synchronized
     fun activeTabId(): String? = tabs.firstOrNull { it.active }?.id
 
     private fun setActive(id: String) {