feat: per-app UA config, fix demo file path and back-loop

app/src/main/java/top/yeij/hearth/MainActivity.kt

@@ -267,13 +267,16 @@ class MainActivity : Activity() {
         private val sidebarWidthPx by lazy {
             (SIDEBAR_WIDTH_DP * resources.displayMetrics.density).toInt()
         }
+        // 加载失败的 webapp id：Back 键时直接关闭而非 goBack 循环回退
+        // Failed webapp ids: on Back, close directly instead of looping goBack
+        private val failedWebViews = mutableSetOf<String>()
 
-        override fun openWebView(id: String, url: String) {
+        override fun openWebView(id: String, url: String, ua: String?) {
             if (contentWebViews.containsKey(id)) {
                 switchWebView(id)
                 return
             }
-            val webView = WebViewManager(this@MainActivity).createContentWebView()
+            val webView = WebViewManager(this@MainActivity).createContentWebView(ua)
             webView.webViewClient = object : WebViewClient() {
                 @Suppress("DEPRECATION", "OVERRIDE_DEPRECATION")
                 override fun onReceivedError(
@@ -283,6 +286,7 @@ class MainActivity : Activity() {
                     failingUrl: String?,
                 ) {
                     Log.d(TAG, "content onReceivedError: code=$code desc=$description url=$failingUrl")
+                    failedWebViews.add(id)
                     view?.loadDataWithBaseURL(null, errorPage(description ?: "未知错误"), "text/html", "utf-8", null)
                 }
             }
@@ -311,6 +315,7 @@ class MainActivity : Activity() {
 
         override fun closeWebView(id: String) {
             val webView = contentWebViews.remove(id) ?: return
+            failedWebViews.remove(id)
             root.removeView(webView)
             webView.stopLoading()
             webView.destroy()
@@ -333,8 +338,12 @@ class MainActivity : Activity() {
         }
 
         override fun goBack(): Boolean {
-            val webView = currentVisibleId?.let { contentWebViews[it] }
-            return if (webView != null && webView.canGoBack()) {
+            val id = currentVisibleId ?: return false
+            // 加载失败的 webapp：Back 直接关闭（返回 false 让 onBackPressed 关闭标签），
+            // 避免 goBack 回退到失败的 file:// 页面反复加载
+            if (failedWebViews.contains(id)) return false
+            val webView = contentWebViews[id] ?: return false
+            return if (webView.canGoBack()) {
                 webView.goBack()
                 true
             } else {