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/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"))
+    }
+}