refactor: split app wrapper from focused example code (single-file apps)

Convert 20 more single-file apps to the validated pattern: lib/main.dart
holds only the runApp + MaterialApp bootstrap, while a new lib/example.dart
holds the full screen (Scaffold, AppBar) and the focused example code.

Apps: enabling_splash_screen, getx_counter_app, image_from_network,
infinite_list, load_local_image, load_local_json, persist_key_value,
push_notifications, sliver_app_bar_example, stateless_widgets, tic_tac_toe,
tip_calculator, using_alert_dialog, using_edittext, using_expansionpanel,
using_http_get, using_interactiveviewer, using_snackbar, using_stepper,
using_theme

tip_calculator/lib/main.dart

@@ -1,73 +1,19 @@
 import 'package:flutter/material.dart';
 
+import 'example.dart';
+
 void main() {
-  runApp(MaterialApp(title: 'Tip Calculator', home: TipCalculator()));
+  runApp(const MyApp());
 }
 
-class TipCalculator extends StatefulWidget {
-  const TipCalculator({super.key});
-
-  @override
-  State<TipCalculator> createState() => _TipCalculatorState();
-}
-
-class _TipCalculatorState extends State<TipCalculator> {
-  double billAmount = 0.0;
-  double tipPercentage = 0.0;
+class MyApp extends StatelessWidget {
+  const MyApp({super.key});
 
   @override
   Widget build(BuildContext context) {
-    // Create first input field
-    TextField billAmountField = TextField(
-      keyboardType: TextInputType.number,
-      onChanged: (String value) {
-        try {
-          billAmount = double.parse(value);
-        } catch (exception) {
-          billAmount = 0.0;
-        }
-      },
-      decoration: InputDecoration(labelText: "Bill amount(\$)"),
+    return MaterialApp(
+      title: 'Tip Calculator',
+      home: const Example(),
     );
-
-    // Create another input field
-    TextField tipPercentageField = TextField(
-        decoration: InputDecoration(labelText: "Tip %", hintText: "15"),
-        keyboardType: TextInputType.number,
-        onChanged: (String value) {
-          try {
-            tipPercentage = double.parse(value);
-          } catch (exception) {
-            tipPercentage = 0.0;
-          }
-        });
-
-    // Create button
-    ElevatedButton calculateButton = ElevatedButton(
-        child: Text("Calculate"),
-        onPressed: () {
-          // Calculate tip and total
-          double calculatedTip = billAmount * tipPercentage / 100.0;
-          double total = billAmount + calculatedTip;
-
-          // Generate dialog
-          AlertDialog dialog = AlertDialog(
-              content: Text("Tip: \$$calculatedTip \n"
-                  "Total: \$$total"));
-
-          // Show dialog
-          showDialog(
-              context: context, builder: (BuildContext context) => dialog);
-        });
-
-    Container container = Container(
-        padding: const EdgeInsets.all(16.0),
-        child: Column(
-            children: [billAmountField, tipPercentageField, calculateButton]));
-
-    AppBar appBar = AppBar(title: Text("Tip Calculator"));
-
-    Scaffold scaffold = Scaffold(appBar: appBar, body: container);
-    return scaffold;
   }
 }