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/src/Dispatcher.php

@@ -117,6 +117,39 @@ class Dispatcher
             };
         }
 
+        // /cover-fragment  (HTMX lazy-load: returns just the cover markup.
+        // Consumed by the home page's `revealed`-triggered placeholders — the
+        // heavy image bytes are fetched only when a card scrolls into view.
+        //
+        // Prefers the small WebP thumbnail (same storage-relative path, .webp
+        // ext) when present, else falls back to the original via <picture>.
+        if ($path === '/cover-fragment' || $path === '/cover-fragment.php') {
+            return static function (): void {
+                $coverPath = $_GET['path'] ?? '';
+
+                // Same strict whitelist as MediaController: no traversal, no junk.
+                if (
+                    $coverPath === ''
+                    || !preg_match('#^[a-zA-Z0-9/_\-.]+$#', $coverPath)
+                ) {
+                    http_response_code(400);
+                    exit;
+                }
+
+                $orig = '/media?path=' . urlencode($coverPath);
+                $webpPath = preg_replace('/\.[a-z0-9]+$/i', '.webp', $coverPath);
+
+                echo '<picture>'
+                    . '<source type="image/webp" srcset="'
+                    . htmlspecialchars('/media?path=' . urlencode((string) $webpPath), ENT_QUOTES, 'UTF-8')
+                    . '">'
+                    . '<img src="'
+                    . htmlspecialchars($orig, ENT_QUOTES, 'UTF-8')
+                    . '" alt="Couverture" loading="lazy">'
+                    . '</picture>';
+            };
+        }
+
         // /maintenance.php
         if ($path === '/maintenance' || $path === '/maintenance.php') {
             return function () {