Return character limit in API and initial state.

app/serializers/initial_state_serializer.rb

@@ -2,10 +2,15 @@
 
 class InitialStateSerializer < ActiveModel::Serializer
   attributes :meta, :compose, :accounts,
-             :media_attachments, :settings, :push_subscription
+             :media_attachments, :settings, :push_subscription,
+             :max_chars
 
   has_many :custom_emojis, serializer: REST::CustomEmojiSerializer
 
+  def max_chars
+    StatusLengthValidator::MAX_CHARS
+  end
+
   def custom_emojis
     CustomEmoji.local.where(disabled: false)
   end

app/serializers/rest/instance_serializer.rb

@@ -4,7 +4,7 @@ class REST::InstanceSerializer < ActiveModel::Serializer
   include RoutingHelper
 
   attributes :uri, :title, :description, :email,
-             :version, :urls, :stats, :thumbnail
+             :version, :urls, :stats, :thumbnail, :max_chars
 
   def uri
     Rails.configuration.x.local_domain
@@ -30,6 +30,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
     full_asset_url(instance_presenter.thumbnail.file.url) if instance_presenter.thumbnail
   end
 
+  def max_chars
+    StatusLengthValidator::MAX_CHARS
+  end
+
   def stats
     {
       user_count: instance_presenter.user_count,