feat: support file and image attachments (#98)

* upload & view attachments to particles

* allow download of attachments

js/src/lib/image-thumbnail.ts

@@ -0,0 +1,24 @@
+/**
+ * Downscale an image file to a thumbnail and return an object URL.
+ * Returns undefined for non-image files.
+ * Caller is responsible for revoking the URL via URL.revokeObjectURL().
+ */
+export async function createImageThumbnail(
+  file: File,
+  maxDim = 200,
+): Promise<string | undefined> {
+  if (!file.type.startsWith("image/")) return undefined;
+
+  const bitmap = await createImageBitmap(file);
+  const scale = Math.min(1, maxDim / Math.max(bitmap.width, bitmap.height));
+  const w = Math.round(bitmap.width * scale);
+  const h = Math.round(bitmap.height * scale);
+
+  const canvas = new OffscreenCanvas(w, h);
+  const ctx = canvas.getContext("2d")!;
+  ctx.drawImage(bitmap, 0, 0, w, h);
+  bitmap.close();
+
+  const blob = await canvas.convertToBlob({ type: "image/jpeg", quality: 0.7 });
+  return URL.createObjectURL(blob);
+}