[birds] Add small js file for form

vrobbler/apps/birds/static/birds/bird_sightings.js

@@ -0,0 +1,27 @@
+document.addEventListener("DOMContentLoaded", function () {
+  var widget = document.querySelector(".bird-sightings-widget");
+  if (!widget) return;
+
+  var list = widget.querySelector(".bird-sightings-list");
+
+  widget.addEventListener("click", function (e) {
+    if (e.target.classList.contains("add-sighting-row")) {
+      var rows = list.querySelectorAll(".bird-sighting-row");
+      var template = rows[rows.length - 1];
+      if (!template) return;
+      var clone = template.cloneNode(true);
+      clone.querySelectorAll("select, input").forEach(function (el) {
+        el.value = "";
+      });
+      clone.querySelector('input[name$="_quantity"]').value = "1";
+      list.appendChild(clone);
+    }
+
+    if (e.target.classList.contains("remove-sighting")) {
+      var rows = list.querySelectorAll(".bird-sighting-row");
+      if (rows.length > 1) {
+        e.target.closest(".bird-sighting-row").remove();
+      }
+    }
+  });
+});