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

app/src/main/java/top/yeij/hearth/webview/WebViewManager.kt

@@ -31,10 +31,18 @@ class WebViewManager(private val activity: Activity) {
     // (lifecycle is managed by the host); transparent background so a webapp without
     // a declared background shows the wallpaper instead of a default white
     @SuppressLint("SetJavaScriptEnabled")
-    fun createContentWebView(): WebView {
+    fun createContentWebView(ua: String?): WebView {
         val webView = WebView(activity)
         webView.setBackgroundColor(Color.TRANSPARENT)
         configure(webView)
+        // UA 解析：desktop/pc → PC UA；自定义字符串 → 直接用；null → 默认平板 UA
+        // UA resolution: desktop/pc -> desktop UA; custom string -> used as-is;
+        // null -> default tablet UA
+        webView.settings.userAgentString = when {
+            ua == null -> UA_TABLET
+            ua == "desktop" || ua == "pc" -> UA_DESKTOP
+            else -> ua
+        }
         return webView
     }
 
@@ -50,4 +58,15 @@ class WebViewManager(private val activity: Activity) {
             domStorageEnabled = true
         }
     }
+
+    companion object {
+        // PC 版 UA（desktop/pc）
+        private const val UA_DESKTOP =
+            "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 " +
+                "(KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36"
+        // 安卓平板 UA（默认）
+        private const val UA_TABLET =
+            "Mozilla/5.0 (Linux; Android 13; SM-X700 Build/TP1A.220624.014) " +
+                "AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36"
+    }
 }