feat(newsletter): Beehiiv signup integration — theme v2.1.7

- Add page-subscribe.php: single-column conversion template for /subscribe/
- Add peptide-newsletter-cta block to single-peptide.php (compact Beehiiv
  iframe at 180px height after article content, before prev/next nav)
- Add peptide_starter_enqueue_beehiiv_attribution() to functions.php:
  loads attribution.js site-wide in footer (one include covers all placements)
- Add 'Get our weekly digest →' footer CTA in column 1 linking to /subscribe/
  (styled accent-green; sits below brand blurb)

Form ID: 6edcd482-cbea-4767-8777-97ac17d6d609
Attribution tracking: https://subscribe-forms.beehiiv.com/attribution.js

page-subscribe.php

@@ -0,0 +1,44 @@
+<?php
+/**
+ * Template Name: Subscribe
+ *
+ * Single-column, signup-form-centric layout for the /subscribe/ landing page.
+ * Lighter than the methodology / how-we-review templates — no sticky two-column ToC.
+ * This is a conversion page, not a reference page.
+ *
+ * Who calls it: WordPress page loader when page_template meta = page-subscribe.php
+ * Dependencies: Beehiiv embed script (loaded inline below); attribution.js loaded
+ *               site-wide via peptide_starter_enqueue_beehiiv_attribution() in functions.php
+ *
+ * @see footer.php — newsletter CTA in footer column 1
+ * @see single-peptide.php — compact inline form on monograph pages
+ * @package peptide-starter
+ */
+
+get_header();
+?>
+
+<main id="main" class="site-main">
+	<div class="page-subscribe">
+		<div class="page-subscribe__content">
+			<?php
+			while ( have_posts() ) {
+				the_post();
+				?>
+				<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
+					<header class="entry-header">
+						<h1 class="entry-title"><?php the_title(); ?></h1>
+					</header>
+					<div class="entry-content">
+						<?php the_content(); ?>
+					</div>
+				</article>
+				<?php
+			}
+			?>
+		</div>
+	</div>
+</main>
+
+<?php
+get_footer();