Kokkoja: first public assembly

Assembled from the development repository by tools/make_release.sh.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client/js/browser_files.js

@@ -0,0 +1,38 @@
+// @ts-check
+/**
+ * Files without the local program: when Kokkoja is opened from a web
+ * site, an image file comes from the browser's own file chooser, and
+ * anything to be saved is a download.
+ */
+
+/**
+ * Let the user choose an image file.
+ * @returns {Promise<File | null>}  Null if none was chosen.
+ */
+export function pickImageFile() {
+  return new Promise((resolve) => {
+    const picker = document.createElement('input');
+    picker.type = 'file';
+    picker.accept = 'image/*';
+    picker.addEventListener('change', () => resolve(picker.files?.[0] ?? null));
+    picker.addEventListener('cancel', () => resolve(null));
+    picker.click();
+  });
+}
+
+/**
+ * Give the user a file, through the browser's downloads.
+ * @param {string} name
+ * @param {Blob} content
+ */
+export function download(name, content) {
+  const address = URL.createObjectURL(content);
+  const link = document.createElement('a');
+  link.href = address;
+  link.download = name;
+  document.body.append(link);
+  link.click();
+  link.remove();
+  // The browser has begun the download; the address can go a moment later.
+  setTimeout(() => URL.revokeObjectURL(address), 10000);
+}