feat: replace mail protocol with smolmail and rebuild the UI

lib/presentation/routes/identity_guard.dart

@@ -0,0 +1,22 @@
+import "package:auto_route/auto_route.dart";
+import "package:flutter_riverpod/flutter_riverpod.dart";
+
+import "package:smol_mail/data/providers/providers.dart";
+import "package:smol_mail/presentation/routes/app_router.gr.dart";
+
+/// Everything past onboarding needs an identity; the account can still be
+/// registered from settings.
+class IdentityGuard extends AutoRouteGuard {
+  final WidgetRef ref;
+
+  IdentityGuard(this.ref);
+
+  @override
+  void onNavigation(NavigationResolver resolver, StackRouter router) {
+    if (ref.read(identityProvider) == null) {
+      router.replace(OnboardingRoute());
+    } else {
+      resolver.next(true);
+    }
+  }
+}