add UnifiedPushAllowedController

lib/controllers/unified_push_allowed.dart

@@ -0,0 +1,33 @@
+import "dart:io";
+
+import "package:flutter_riverpod/flutter_riverpod.dart";
+import "package:nexus/controllers/client.dart";
+import "package:unifiedpush/unifiedpush.dart";
+
+class UnifiedPushAllowedController extends AsyncNotifier<String?> {
+  @override
+  Future<String?> build() async {
+    if (!await UnifiedPush.tryUseCurrentOrDefaultDistributor()) {
+      return "No valid distributors found. Try installing ${Platform.isLinux
+          ? "KUnifiedPush"
+          : Platform.isAndroid
+          ? "FCM or NTFY"
+          : "NTFY"}.";
+    }
+
+    final capabilities = await ref
+        .watch(ClientController.provider.notifier)
+        .getCapabilities();
+
+    if (capabilities.webpush?.vapid == null) {
+      return "Your homeserver does not support MSC4174 (Web Push), and therefore cannot send notifications to Nexus.";
+    }
+
+    return null;
+  }
+
+  static final provider =
+      AsyncNotifierProvider.autoDispose<UnifiedPushAllowedController, String?>(
+        UnifiedPushAllowedController.new,
+      );
+}