web: Implement drag-and-drop upload

assets/css/main.css

@@ -0,0 +1,18 @@
+body {
+    font-family: monospace;
+}
+
+#dropZone {
+    background-color: rgba(0, 0, 0.2, 0.75);
+    color: white;
+    font-size: 60px;
+    font-weight: bold;
+    height: 100%;         
+    left: 0; 
+    padding: 1em;
+    position: fixed;
+    top: 0;
+    transition: visibility 175ms, opacity 175ms;
+    width: 100%;
+    z-index: 999;               
+}