revert: restore conversation unread count feature flag (#14623)

This reverts #14610 so conversation unread counts are again controlled
by the `conversation_unread_counts` feature flag across the API,
ActionCable broadcasts, notifier/listener paths, and dashboard sidebar
fetching.

## Closes
- None

## What changed
- Restores feature-flag checks for conversation unread count reads and
broadcasts.
- Restores the dashboard feature flag constant and sidebar/store
behavior for disabled unread counts.
- Restores the specs that cover disabled-feature behavior.

## How to test
- In an account with `conversation_unread_counts` enabled, verify
sidebar unread counts are fetched and updated in real time.
- Disable `conversation_unread_counts` for the account and verify unread
count requests/broadcasts are skipped.

app/controllers/api/v1/accounts/conversations/unread_counts_controller.rb

@@ -1,6 +1,16 @@
 class Api::V1::Accounts::Conversations::UnreadCountsController < Api::V1::Accounts::BaseController
+  before_action :ensure_unread_counts_enabled
+
   def index
     counts = ::Conversations::UnreadCounts::Counter.new(account: Current.account, user: Current.user).perform
     render json: { payload: counts }
   end
+
+  private
+
+  def ensure_unread_counts_enabled
+    return if Current.account.feature_enabled?('conversation_unread_counts')
+
+    render json: { error: I18n.t('errors.conversations.unread_counts.feature_not_enabled') }, status: :forbidden
+  end
 end