fix: add focus states for keyboard a11y and guard empty excerpt

Addresses QA Agent 2 findings:
- Add :focus outline on title and thumbnail links for keyboard navigation
- Wrap excerpt paragraph in conditional to avoid empty <p> when both
  ai_summary and excerpt are missing

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

public/class-peptide-news-public.php

@@ -130,13 +130,14 @@ class Peptide_News_Public {
                             </a>
                         </h3>
 
-                        <p class="pn-article-excerpt">
                         <?php
                         // Prefer AI summary over raw excerpt.
-                            $display_text = ! empty( $article->ai_summary ) ? $article->ai_summary : $article->excerpt;
+                        $display_text = ! empty( $article->ai_summary ) ? $article->ai_summary : ( $article->excerpt ?? '' );
-                            echo esc_html( $display_text );
+                        if ( ! empty( $display_text ) ) : ?>
-                            ?>
+                            <p class="pn-article-excerpt">
+                                <?php echo esc_html( $display_text ); ?>
                             </p>
+                        <?php endif; ?>
 
                         <?php if ( ! empty( $article->author ) ) : ?>
                             <span class="pn-author">

public/css/public-style.css

@@ -121,6 +121,12 @@
     color: #0073aa;
 }
 
+.pn-article-title a:focus,
+.pn-article-thumb a:focus {
+    outline: 2px solid #0073aa;
+    outline-offset: 2px;
+}
+
 /* ── Excerpt / AI summary ──────────────────────────────────────── */
 .pn-article-excerpt {
     font-size: 0.875rem;