new: add offline fallback support for WebView

shared/src/iosMain/kotlin/bzh/ajaury/chombev/web/ui/PlatformWebView.ios.kt

@@ -23,6 +23,7 @@ import platform.darwin.NSObject
 @Composable
 actual fun PlatformWebView(
     url: String,
+    offlineHtml: String?,
     darkTheme: Boolean,
     onLoadingStateChange: (Boolean) -> Unit,
     modifier: Modifier,
@@ -30,6 +31,8 @@ actual fun PlatformWebView(
     // WKWebView holds its navigationDelegate weakly, so keep a strong reference across recompositions.
     val navigationDelegate = remember { WebViewNavigationDelegate() }
     navigationDelegate.onLoadingStateChange = onLoadingStateChange
+    navigationDelegate.offlineHtml = offlineHtml
+    navigationDelegate.baseUrl = url
 
     UIKitView(
         modifier = modifier,
@@ -54,12 +57,19 @@ actual fun PlatformWebView(
 
 /**
  * Forwards WKWebView load lifecycle events to [onLoadingStateChange] so the UI can show a loader
- * while a page is loading and hide it once loading finishes (or fails).
+ * while a page is loading and hide it once loading finishes.
+ *
+ * When a navigation fails, it swaps in [offlineHtml] (loaded against [baseUrl] so links resolve) so
+ * the page still shows content offline. [showingOffline] guards against re-entering the fallback.
  */
 private class WebViewNavigationDelegate :
     NSObject(),
     WKNavigationDelegateProtocol {
     var onLoadingStateChange: (Boolean) -> Unit = {}
+    var offlineHtml: String? = null
+    var baseUrl: String = ""
+
+    private var showingOffline = false
 
     override fun webView(
         webView: WKWebView,
@@ -82,7 +92,7 @@ private class WebViewNavigationDelegate :
         didFailNavigation: WKNavigation?,
         withError: NSError,
     ) {
-        onLoadingStateChange(false)
+        handleFailure(webView)
     }
 
     // Shares the (WKWebView, WKNavigation?, NSError) Kotlin signature with didFailNavigation above.
@@ -92,7 +102,17 @@ private class WebViewNavigationDelegate :
         didFailProvisionalNavigation: WKNavigation?,
         withError: NSError,
     ) {
-        onLoadingStateChange(false)
+        handleFailure(webView)
+    }
+
+    private fun handleFailure(webView: WKWebView) {
+        val html = offlineHtml
+        if (html != null && !showingOffline) {
+            showingOffline = true
+            webView.loadHTMLString(string = html, baseURL = NSURL.URLWithString(baseUrl))
+        } else {
+            onLoadingStateChange(false)
+        }
     }
 }