add send test email button to campaign form

Sends the campaign to the admin's own email address as a preview,
with [Test] prefix in subject line. Uses the same HTML template
and formatting as real sends. Does not affect campaign status or
sent counts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod/newsletter/notifier.ex

@@ -42,6 +42,27 @@ defmodule Berrypod.Newsletter.Notifier do
     deliver(subscriber.email, "Confirm your subscription", text, html)
   end
 
+  @doc "Sends a test/preview email of a campaign to the given address."
+  def deliver_test(campaign, to_email) do
+    shop_name = Berrypod.Settings.get_setting("shop_name", "Berrypod")
+    sample_unsub = BerrypodWeb.Endpoint.url() <> "/unsubscribe/test-preview"
+
+    body_with_url =
+      String.replace(campaign.body, "{{unsubscribe_url}}", sample_unsub)
+
+    text = """
+    [TEST] #{body_with_url}
+
+    ---
+    This is a test email. No subscribers received this.
+    """
+
+    html_content = text_to_html(body_with_url, sample_unsub)
+    html = wrap_html(shop_name, html_content, sample_unsub)
+
+    deliver(to_email, "[Test] #{campaign.subject}", text, html)
+  end
+
   @doc "Sends a campaign email to a single subscriber."
   def deliver_campaign(campaign, subscriber) do
     shop_name = Berrypod.Settings.get_setting("shop_name", "Berrypod")