feat(home): htmx lazy-load cover images

Replace the eager <img> on the home page with an htmx placeholder <figure>
that fetches a /cover-fragment endpoint when it scrolls into view
(hx-trigger="revealed"), so heavy cover bytes load only on demand.
Add spinner + settle-fade transition CSS, and load htmx.min.js on home.

app/templates/public/home.php

@@ -17,10 +17,14 @@
                     $thumb = $coverMap[$item['id']] ?? null;
                     ?>
                     <?php if ($thumb): ?>
-                        <figure>
-                            <img src="/media?path=<?= urlencode($thumb) ?>"
-                                 alt="Couverture — <?= htmlspecialchars($item['title']) ?> par <?= htmlspecialchars($item['authors'] ?? '') ?>"
-                                 loading="lazy">
+                        <figure
+                            hx-get="/cover-fragment?path=<?= urlencode($thumb) ?>"
+                            hx-trigger="intersect threshold:0.05 once"
+                            hx-swap="innerHTML"
+                            aria-busy="true">
+                            <div class="card__media--placeholder" role="status">
+                                <span class="card__cover-loading">…</span>
+                            </div>
                         </figure>
                     <?php else: ?>
                         <div class="card__media--gradient"