feat: Add network error handling

- error.html: User-friendly error page with retry button
- ErrorHandler.kt: Utility for handling WebView errors
- ErrorHandlerTest.kt: Unit tests for error handling

Shows friendly error page on network failures instead of blank/broken page.
Includes error code display and one-tap retry.

app/src/main/assets/error.html

@@ -0,0 +1,88 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <title>Connection Error</title>
+    <style>
+        * {
+            margin: 0;
+            padding: 0;
+            box-sizing: border-box;
+        }
+        body {
+            font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, sans-serif;
+            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
+            min-height: 100vh;
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            padding: 20px;
+        }
+        .error-container {
+            background: white;
+            border-radius: 16px;
+            padding: 40px;
+            text-align: center;
+            max-width: 400px;
+            box-shadow: 0 10px 40px rgba(0,0,0,0.2);
+        }
+        .icon {
+            font-size: 64px;
+            margin-bottom: 20px;
+        }
+        h1 {
+            color: #333;
+            font-size: 24px;
+            margin-bottom: 12px;
+        }
+        p {
+            color: #666;
+            font-size: 16px;
+            line-height: 1.5;
+            margin-bottom: 24px;
+        }
+        .retry-btn {
+            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
+            color: white;
+            border: none;
+            padding: 14px 32px;
+            font-size: 16px;
+            border-radius: 8px;
+            cursor: pointer;
+            transition: transform 0.2s, box-shadow 0.2s;
+        }
+        .retry-btn:hover {
+            transform: translateY(-2px);
+            box-shadow: 0 4px 12px rgba(102, 126, 234, 0.4);
+        }
+        .retry-btn:active {
+            transform: translateY(0);
+        }
+        .error-code {
+            margin-top: 20px;
+            font-size: 12px;
+            color: #999;
+        }
+    </style>
+</head>
+<body>
+    <div class="error-container">
+        <div class="icon">📡</div>
+        <h1>Connection Problem</h1>
+        <p>Unable to load the page. Please check your internet connection and try again.</p>
+        <button class="retry-btn" onclick="window.location.reload()">
+            Try Again
+        </button>
+        <div class="error-code" id="errorCode"></div>
+    </div>
+    <script>
+        // Parse error code from URL if present
+        const params = new URLSearchParams(window.location.search);
+        const code = params.get('code');
+        if (code) {
+            document.getElementById('errorCode').textContent = 'Error code: ' + code;
+        }
+    </script>
+</body>
+</html>

app/src/main/kotlin/farm/hobbs/kioskbrowser/ErrorHandler.kt

@@ -0,0 +1,80 @@
+package farm.hobbs.kioskbrowser
+
+import android.webkit.WebResourceError
+import android.webkit.WebResourceRequest
+import android.webkit.WebView
+
+/**
+ * Handles WebView errors with user-friendly error pages.
+ * 
+ * Usage in WebViewClient:
+ * ```
+ * override fun onReceivedError(view: WebView?, request: WebResourceRequest?, error: WebResourceError?) {
+ *     ErrorHandler.handleError(view, request, error)
+ * }
+ * ```
+ */
+object ErrorHandler {
+
+    private const val ERROR_PAGE = "file:///android_asset/error.html"
+
+    /**
+     * Network error codes that should show the error page
+     */
+    private val NETWORK_ERRORS = setOf(
+        android.webkit.WebViewClient.ERROR_HOST_LOOKUP,
+        android.webkit.WebViewClient.ERROR_CONNECT,
+        android.webkit.WebViewClient.ERROR_TIMEOUT,
+        android.webkit.WebViewClient.ERROR_IO,
+        android.webkit.WebViewClient.ERROR_UNKNOWN
+    )
+
+    /**
+     * Handle WebView errors - show error page for main frame network errors
+     */
+    fun handleError(
+        view: WebView?,
+        request: WebResourceRequest?,
+        error: WebResourceError?
+    ): Boolean {
+        // Only handle main frame errors
+        if (request?.isForMainFrame != true) return false
+        
+        val errorCode = error?.errorCode ?: return false
+        
+        // Show error page for network errors
+        if (errorCode in NETWORK_ERRORS || errorCode < 0) {
+            view?.loadUrl("$ERROR_PAGE?code=$errorCode")
+            return true
+        }
+        
+        return false
+    }
+
+    /**
+     * Handle HTTP errors (4xx, 5xx)
+     */
+    fun handleHttpError(
+        view: WebView?,
+        request: WebResourceRequest?,
+        statusCode: Int
+    ): Boolean {
+        // Only handle main frame errors
+        if (request?.isForMainFrame != true) return false
+        
+        // Show error page for server errors
+        if (statusCode >= 500) {
+            view?.loadUrl("$ERROR_PAGE?code=$statusCode")
+            return true
+        }
+        
+        return false
+    }
+
+    /**
+     * Check if current URL is the error page
+     */
+    fun isErrorPage(url: String?): Boolean {
+        return url?.startsWith(ERROR_PAGE.substringBefore("?")) == true
+    }
+}

app/src/test/kotlin/farm/hobbs/kioskbrowser/ErrorHandlerTest.kt

@@ -0,0 +1,29 @@
+package farm.hobbs.kioskbrowser
+
+import org.junit.Assert.*
+import org.junit.Test
+
+/**
+ * Unit tests for ErrorHandler utility
+ */
+class ErrorHandlerTest {
+
+    @Test
+    fun `isErrorPage returns true for error page URL`() {
+        assertTrue(ErrorHandler.isErrorPage("file:///android_asset/error.html"))
+        assertTrue(ErrorHandler.isErrorPage("file:///android_asset/error.html?code=-2"))
+    }
+
+    @Test
+    fun `isErrorPage returns false for regular URLs`() {
+        assertFalse(ErrorHandler.isErrorPage("https://hobbs.farm"))
+        assertFalse(ErrorHandler.isErrorPage("https://google.com"))
+        assertFalse(ErrorHandler.isErrorPage(null))
+    }
+
+    @Test
+    fun `isErrorPage returns false for similar but different URLs`() {
+        assertFalse(ErrorHandler.isErrorPage("file:///android_asset/other.html"))
+        assertFalse(ErrorHandler.isErrorPage("https://error.html"))
+    }
+}