Export module for a website gallery with selected images

This export module should replace the PhotoSwipe copy in darktable.

See https://github.com/darktable-org/darktable/issues/16205 for more
details.

data/website_gallery/modal.css

@@ -0,0 +1,95 @@
+* {
+    margin: 0;
+    padding: 0;
+    box-sizing: border-box;
+}
+body {
+    margin: 0;
+    padding: 0;
+    width: 100vw;
+    height: 100vh;
+    background: #000;
+    display: flex;
+    flex-direction: column;
+}
+
+.viewer {
+    position: relative;
+    width: 100vw;
+    height: 100vh;
+    touch-action: none;
+    overflow: hidden;
+    display: none;
+}
+
+.slider {
+    position: relative;
+    width: 100vw;
+    height: 95vh;
+    touch-action: none;
+    overflow: hidden;
+}
+
+.slide-container {
+    position: absolute;
+    top: 0;
+    left: 0;
+    width: 300%;
+    height: 100%;
+    display: flex;
+    transform: translateX(-33.333%);
+    transition: transform 0.3s ease-out;
+    will-change: transform;
+}
+
+.slide {
+    width: 33.333%;
+    height: 100%;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    background: #000;
+    will-change: contents;
+    overflow: hidden;
+    position: relative;
+    padding: 0;
+    margin: 0;
+    touch-action: none;
+    object-fit: contain;
+}
+
+.slide img {
+    max-width: 100%;
+    max-height: 100%;
+    width: auto;
+    height: auto;
+    padding: 0;
+    margin: 0;
+    display: block;
+    transform-origin: 0 0;
+    cursor: zoom-in;
+    object-fit: contain;
+}
+
+.slide.zoomed img {
+    cursor: zoom-out;
+    max-width: none;
+    max-height: none;
+}
+
+.slide img.zooming {
+    transition: transform 0.2s ease-out;
+}
+
+.nav-bar {
+    width: 100vw;
+    height: 5vh;
+    background: #333;
+    color: #fff;
+    display: flex;
+    align-items: center;
+    padding-left: 20px;
+    box-sizing: border-box;
+    font-family: Arial, sans-serif;
+    font-size: 16px;
+}