Add warning when connected to demo server

lib/main.dart

@@ -88,6 +88,7 @@ class _MyHomePageState extends State<MyHomePage> {
   var _fetched = false;
   var _fetchedCount = 0;
   var _knownPostsSize = -1;
+  var _isDemoUser = false;
   List<int> _feedFilter = [];
 
   Future<void> refresh() async {
@@ -136,6 +137,11 @@ class _MyHomePageState extends State<MyHomePage> {
   @override
   void initState() {
     super.initState();
+
+    setState(() {
+      _isDemoUser = widget.prefs.getBool("isDemo") ?? false;
+    });
+
     refresh().then((_) => setState(() {
           _fetched = true;
         }));
@@ -204,6 +210,47 @@ class _MyHomePageState extends State<MyHomePage> {
       appBar: AppBar(
         title: Text(widget.title),
         actions: [
+          _isDemoUser
+            ? ElevatedButton(
+              onPressed: () {
+                showDialog(
+                  context: context,
+                  builder: (context) => AlertDialog(
+                    title: const Text("Connected to demo server"),
+                    content: RichText(text: TextSpan(
+                      children: [
+                        const TextSpan(
+                          text: "You are currently connected to the demo server.\n"
+                          "Use the"
+                        ),
+                        WidgetSpan(child: Icon(Icons.adaptive.more, size: 20),
+                        ),
+                        const TextSpan(
+                          text: "button in the top right to log out and switch "
+                                "to your own instance once you're done exploring."
+                        ),
+                      ],
+                    )),
+                    actions: [
+                      TextButton(
+                        onPressed: () => Navigator.of(context).pop(),
+                        child: const Text("Got it!"),
+                      ),
+                    ],
+                  ),
+                );
+              },
+              style: ElevatedButton.styleFrom(
+                backgroundColor: const Color.fromARGB(255, 114, 77, 7),
+                foregroundColor: Colors.amber,
+              ),
+              child: Row(children: const [
+                Icon(Icons.warning),
+                SizedBox(width: 8),
+                Text("Demo"),
+              ])
+            )
+            : Container(),
           IconButton(
             onPressed: () {
               var theme = Theme.of(context);
@@ -290,7 +337,8 @@ class _MyHomePageState extends State<MyHomePage> {
                       value: "feedback",
                       child: const Text("Feedback"),
                       onTap: () async {
-                        await openUrl(context, "https://git.amogus.cloud/Lea/feet/issues");
+                        await openUrl(context,
+                            "https://git.amogus.cloud/Lea/feet/issues");
                       },
                     ),
                   ]),