Public: defer category image fetch and unblock card navigation

Avoid navigation stalls on cold loads by delaying thumbnail requests until cards are near viewport. Stop in-flight page downloads on card click so opening a photo page is immediate even while list images are still loading.

index.php

@@ -239,7 +239,7 @@ function outputWatermarked(string $path, string $mime): never
             <div class="cards">
               <?php foreach($photos as $p): ?>
                 <a class="card" href="?photo_id=<?= (int)$p['id'] ?>&section_id=<?= (int)$activeSectionId ?><?= $viewerToken!=='' ? '&viewer=' . urlencode($viewerToken) : '' ?>" style="text-decoration:none;color:inherit;position:relative">
-                  <?php if (!empty($p['before_file_id'])): ?><div class="img-box thumb-img-box is-loading"><img class="js-public-image" src="?action=image&file_id=<?= (int)$p['before_file_id'] ?>" alt="" loading="lazy" decoding="async" fetchpriority="low"></div><?php endif; ?>
+                  <?php if (!empty($p['before_file_id'])): ?><div class="img-box thumb-img-box is-loading"><img class="js-public-image js-card-image" src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==" data-src="?action=image&file_id=<?= (int)$p['before_file_id'] ?>" alt="" loading="lazy" decoding="async" fetchpriority="low"></div><?php endif; ?>
                   <?php if (!empty($p['after_file_id'])): ?><span title="Есть обработанная версия" style="position:absolute;top:8px;right:8px;background:rgba(31,111,235,.92);color:#fff;font-size:11px;line-height:1;padding:6px 7px;border-radius:999px">AI</span><?php endif; ?>
                   <div class="cap"><strong><?= h((string)$p['code_name']) ?></strong><br><span class="muted"><?= h((string)($p['description'] ?? '')) ?></span></div>
                 </a>
@@ -257,6 +257,34 @@ function outputWatermarked(string $path, string $mime): never
 </div>
 <script>
 (() => {
+  const loadCardImage = (img) => {
+    const src = img.dataset.src;
+    if (!src) {
+      return;
+    }
+
+    img.src = src;
+    img.removeAttribute('data-src');
+  };
+
+  const deferred = document.querySelectorAll('.js-card-image[data-src]');
+  if ('IntersectionObserver' in window) {
+    const io = new IntersectionObserver((entries) => {
+      entries.forEach((entry) => {
+        if (!entry.isIntersecting) {
+          return;
+        }
+
+        loadCardImage(entry.target);
+        io.unobserve(entry.target);
+      });
+    }, { rootMargin: '240px 0px' });
+
+    deferred.forEach((img) => io.observe(img));
+  } else {
+    deferred.forEach((img) => loadCardImage(img));
+  }
+
   document.querySelectorAll('img').forEach((img) => {
     img.addEventListener('contextmenu', (e) => e.preventDefault());
     img.addEventListener('dragstart', (e) => e.preventDefault());
@@ -275,10 +303,27 @@ function outputWatermarked(string $path, string $mime): never
     img.addEventListener('load', clearLoading, { once: true });
     img.addEventListener('error', clearLoading, { once: true });
 
-    if (img.complete) {
+    if (img.complete && !img.dataset.src) {
       clearLoading();
     }
   });
+
+  document.querySelectorAll('a.card').forEach((link) => {
+    link.addEventListener('click', (e) => {
+      if (e.defaultPrevented || e.button !== 0 || e.metaKey || e.ctrlKey || e.shiftKey || e.altKey) {
+        return;
+      }
+
+      const href = link.getAttribute('href');
+      if (!href) {
+        return;
+      }
+
+      e.preventDefault();
+      window.stop();
+      window.location.assign(href);
+    });
+  });
 })();
 
 (() => {