Add server-generated URL preview support

lib/controllers/url_preview_controller.dart

@@ -0,0 +1,63 @@
+import "dart:convert";
+import "package:flutter_chat_core/flutter_chat_core.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:http/http.dart";
+import "package:nexus/controllers/client_state_controller.dart";
+import "package:nexus/controllers/header_controller.dart";
+import "package:nexus/helpers/extensions/mxc_to_https.dart";
+
+class UrlPreviewController extends AsyncNotifier<LinkPreviewData?> {
+  final TextMessage message;
+  UrlPreviewController(this.message);
+
+  @override
+  Future<LinkPreviewData?> build() async {
+    final homeserver = ref.watch(ClientStateController.provider)?.homeserverUrl;
+    final link = RegExp(
+      r'''https?://[^\s"'<>]+''',
+    ).allMatches(message.text).firstOrNull?.group(0);
+
+    if (homeserver != null && link != null) {
+      {
+        final response = await get(
+          Uri.parse(homeserver)
+              .resolve("/_matrix/client/v1/media/preview_url")
+              .replace(queryParameters: {"url": link}),
+          headers: await ref.watch(HeaderController.provider.future),
+        );
+
+        if (response.statusCode == 200) {
+          final decodedValue = json.decode(response.body);
+          final mxc = decodedValue["og:image"];
+          final image = mxc == null
+              ? null
+              : Uri.tryParse(mxc)?.mxcToHttps(homeserver);
+
+          return LinkPreviewData(
+            link: link,
+            title: decodedValue["og:title"],
+            description: decodedValue["og:description"],
+            image: image == null
+                ? null
+                : ImagePreviewData(
+                    url: image.toString(),
+                    width:
+                        (decodedValue["og:image:width"] as int?)?.toDouble() ??
+                        0,
+                    height:
+                        (decodedValue["og:image:height"] as int?)?.toDouble() ??
+                        0,
+                  ),
+          );
+        }
+      }
+    }
+
+    return null;
+  }
+
+  static final provider = AsyncNotifierProvider.autoDispose
+      .family<UrlPreviewController, LinkPreviewData?, TextMessage>(
+        UrlPreviewController.new,
+      );
+}