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/static/style.css

@@ -326,3 +326,68 @@ h2 {
   margin-top: 1.5rem;
   cursor: pointer;
 }
+
+.variant-actions {
+  display: flex;
+  flex-wrap: wrap;
+  align-items: center;
+  gap: 0.5rem 0.75rem;
+  margin-top: 0.35rem;
+}
+
+.variant-actions .button {
+  padding: 0.35rem 0.75rem;
+  min-height: 2.25rem;
+  line-height: 1.4rem;
+  font-size: 0.85rem;
+}
+
+img.lightboxable {
+  cursor: zoom-in;
+}
+
+/* --- lightbox --- */
+
+body.lightbox-open {
+  overflow: hidden;
+}
+
+.lightbox {
+  position: fixed;
+  inset: 0;
+  z-index: 1000;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 1rem;
+  background: rgba(0, 0, 0, 0.92);
+}
+
+.lightbox[hidden] {
+  display: none;
+}
+
+.lightbox img {
+  max-width: 100%;
+  max-height: 100%;
+  width: auto;
+  height: auto;
+  object-fit: contain;
+  aspect-ratio: auto;
+  background: transparent;
+}
+
+.lightbox-close {
+  position: absolute;
+  top: 0.5rem;
+  right: 0.75rem;
+  border: 0;
+  background: transparent;
+  color: var(--text);
+  font-size: 2rem;
+  line-height: 1;
+  cursor: pointer;
+  padding: 0.25rem 0.5rem;
+  min-height: 2.75rem;
+  min-width: 2.75rem;
+}