feat: harden pipeline UX — preprocess, rembg alpha, remix/lightbox

Downscale to 2000px JPEG before rembg, random blend opacity, timeout
retries, per-variant remix prefill, lightbox, and longer SFTP idle.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/templates/base.html

@@ -13,5 +13,50 @@
   <main>
     {% block content %}{% endblock %}
   </main>
+
+  <div id="lightbox" class="lightbox" hidden>
+    <button type="button" class="lightbox-close" aria-label="Schliessen">&times;</button>
+    <img id="lightbox-img" alt="">
+  </div>
+
+  <script>
+  (function () {
+    var box = document.getElementById("lightbox");
+    var img = document.getElementById("lightbox-img");
+    if (!box || !img) return;
+
+    function openLightbox(src, alt) {
+      img.src = src;
+      img.alt = alt || "";
+      box.hidden = false;
+      document.body.classList.add("lightbox-open");
+    }
+
+    function closeLightbox() {
+      box.hidden = true;
+      img.removeAttribute("src");
+      document.body.classList.remove("lightbox-open");
+    }
+
+    document.addEventListener("click", function (e) {
+      var target = e.target;
+      if (!(target instanceof HTMLImageElement)) return;
+      if (!target.classList.contains("lightboxable")) return;
+      e.preventDefault();
+      e.stopPropagation();
+      openLightbox(target.currentSrc || target.src, target.alt);
+    });
+
+    box.addEventListener("click", function (e) {
+      if (e.target === box || (e.target && e.target.classList.contains("lightbox-close"))) {
+        closeLightbox();
+      }
+    });
+
+    document.addEventListener("keydown", function (e) {
+      if (e.key === "Escape" && !box.hidden) closeLightbox();
+    });
+  })();
+  </script>
 </body>
 </html>