add HTML email templates for newsletter

Multipart emails (HTML + plain text fallback) with a branded wrapper:
shop name header, content area with auto-linked URLs and paragraph
formatting, and unsubscribe footer. Applied to both confirmation and
campaign emails.

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

lib/berrypod/newsletter/notifier.ex

@@ -1,6 +1,9 @@
 defmodule Berrypod.Newsletter.Notifier do
   @moduledoc """
-  Plain text email templates for newsletter confirmation and campaigns.
+  Email delivery for newsletter confirmation and campaigns.
+
+  Sends multipart emails (HTML + plain text fallback) wrapped in a
+  simple branded template with the shop name and unsubscribe footer.
   """
 
   import Swoosh.Email
@@ -14,9 +17,7 @@ defmodule Berrypod.Newsletter.Notifier do
     shop_name = Berrypod.Settings.get_setting("shop_name", "Berrypod")
     confirm_url = BerrypodWeb.Endpoint.url() <> "/newsletter/confirm/" <> raw_token
 
-    body = """
-    ==============================
-
+    text = """
     Thanks for signing up to the #{shop_name} newsletter!
 
     Please confirm your email to start receiving updates:
@@ -25,11 +26,20 @@ defmodule Berrypod.Newsletter.Notifier do
 
     This link expires in 48 hours. If you didn't subscribe,
     just ignore this email and you won't hear from us.
-
-    ==============================
     """
 
-    deliver(subscriber.email, "Confirm your subscription", body)
+    html_content = """
+    <p>Thanks for signing up to the #{esc(shop_name)} newsletter!</p>
+    <p>Please confirm your email to start receiving updates:</p>
+    <p style="margin: 24px 0;">
+      <a href="#{esc(confirm_url)}" style="display: inline-block; padding: 12px 24px; background-color: #111827; color: #ffffff; text-decoration: none; border-radius: 6px; font-weight: 600;">Confirm subscription</a>
+    </p>
+    <p style="color: #6b7280; font-size: 14px;">This link expires in 48 hours. If you didn't subscribe, just ignore this email.</p>
+    """
+
+    html = wrap_html(shop_name, html_content)
+
+    deliver(subscriber.email, "Confirm your subscription", text, html)
   end
 
   @doc "Sends a campaign email to a single subscriber."
@@ -40,7 +50,7 @@ defmodule Berrypod.Newsletter.Notifier do
     body_with_url =
       String.replace(campaign.body, "{{unsubscribe_url}}", unsubscribe_url)
 
-    full_body = """
+    text = """
     #{body_with_url}
 
     ---
@@ -48,12 +58,16 @@ defmodule Berrypod.Newsletter.Notifier do
     Unsubscribe: #{unsubscribe_url}
     """
 
+    html_content = text_to_html(body_with_url, unsubscribe_url)
+    html = wrap_html(shop_name, html_content, unsubscribe_url)
+
     email =
       new()
       |> to(subscriber.email)
       |> from({shop_name, from_address()})
       |> subject(campaign.subject)
-      |> text_body(full_body)
+      |> text_body(text)
+      |> html_body(html)
       |> header("List-Unsubscribe", "<#{unsubscribe_url}>")
       |> header("List-Unsubscribe-Post", "List-Unsubscribe=One-Click")
 
@@ -67,7 +81,7 @@ defmodule Berrypod.Newsletter.Notifier do
     end
   end
 
-  defp deliver(recipient, subject, body) do
+  defp deliver(recipient, subject, text, html) do
     shop_name = Berrypod.Settings.get_setting("shop_name", "Berrypod")
 
     email =
@@ -75,7 +89,8 @@ defmodule Berrypod.Newsletter.Notifier do
       |> to(recipient)
       |> from({shop_name, from_address()})
       |> subject(subject)
-      |> text_body(body)
+      |> text_body(text)
+      |> html_body(html)
 
     case Mailer.deliver(email) do
       {:ok, _metadata} = result ->
@@ -87,6 +102,89 @@ defmodule Berrypod.Newsletter.Notifier do
     end
   end
 
+  # Converts plain text campaign body to HTML paragraphs.
+  # Splits on blank lines for paragraphs, preserves single line breaks.
+  @doc false
+  def text_to_html(text, unsubscribe_url \\ nil) do
+    text
+    |> String.replace("{{unsubscribe_url}}", unsubscribe_url || "#")
+    |> String.split(~r/\n{2,}/, trim: true)
+    |> Enum.map(fn paragraph ->
+      paragraph
+      |> String.trim()
+      |> esc()
+      |> linkify()
+      |> String.replace("\n", "<br>")
+      |> then(&"<p>#{&1}</p>")
+    end)
+    |> Enum.join("\n")
+  end
+
+  # Wraps HTML content in a branded email template.
+  # Keeps it simple: single column, shop name header, optional unsubscribe footer.
+  @doc false
+  def wrap_html(shop_name, content, unsubscribe_url \\ nil) do
+    footer =
+      if unsubscribe_url do
+        """
+        <p style="color: #9ca3af; font-size: 12px; margin-top: 32px; padding-top: 16px; border-top: 1px solid #e5e7eb;">
+          You're receiving this because you subscribed to the #{esc(shop_name)} newsletter.<br>
+          <a href="#{esc(unsubscribe_url)}" style="color: #9ca3af;">Unsubscribe</a>
+        </p>
+        """
+      else
+        ""
+      end
+
+    """
+    <!DOCTYPE html>
+    <html lang="en">
+    <head>
+      <meta charset="utf-8">
+      <meta name="viewport" content="width=device-width, initial-scale=1.0">
+      <title>#{esc(shop_name)}</title>
+    </head>
+    <body style="margin: 0; padding: 0; background-color: #f3f4f6; font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;">
+      <table role="presentation" width="100%" style="background-color: #f3f4f6;">
+        <tr>
+          <td align="center" style="padding: 32px 16px;">
+            <table role="presentation" width="100%" style="max-width: 560px; background-color: #ffffff; border-radius: 8px; overflow: hidden;">
+              <tr>
+                <td style="padding: 24px 32px 16px; border-bottom: 1px solid #e5e7eb;">
+                  <strong style="font-size: 18px; color: #111827;">#{esc(shop_name)}</strong>
+                </td>
+              </tr>
+              <tr>
+                <td style="padding: 24px 32px 32px; color: #374151; font-size: 16px; line-height: 1.6;">
+                  #{content}
+                  #{footer}
+                </td>
+              </tr>
+            </table>
+          </td>
+        </tr>
+      </table>
+    </body>
+    </html>
+    """
+  end
+
+  # Turns bare URLs in escaped text into clickable links.
+  defp linkify(escaped_text) do
+    Regex.replace(~r/(https?:\/\/[^\s<]+)/, escaped_text, fn url, _ ->
+      ~s(<a href="#{url}" style="color: #2563eb;">#{url}</a>)
+    end)
+  end
+
+  # HTML-escapes a string for safe embedding.
+  defp esc(text) do
+    text
+    |> String.replace("&", "&amp;")
+    |> String.replace("<", "&lt;")
+    |> String.replace(">", "&gt;")
+    |> String.replace("\"", "&quot;")
+  end
+
   defp from_address do
     Berrypod.Settings.get_setting("email_from_address", "noreply@example.com")
   end