Support image blur placeholders during development (#3234)

## Description

This is a quality-of-life change that adds an image placeholder (though
not derived from the original image) during development.

Before:


https://github.com/user-attachments/assets/9b9ea1cf-499b-4411-8209-7bb2dba75a4d



After:


https://github.com/user-attachments/assets/45cde166-ee16-40da-9d35-076ba18169b1




## Checklist
- [x] I have read and understood the [WATcloud
Guidelines](https://cloud.watonomous.ca/docs/community-docs/watcloud/guidelines)
- [x] I have performed a self-review of my code

components/picture.tsx

@@ -1,5 +1,6 @@
 import type { WATcloudStaticImage } from "@/build/fixtures/images";
 import { getImageBlurSvg } from "@/lib/image-blur-svg";
+import { shimmer, toBase64 } from "@/lib/utils";
 
 /**
  * Renders an image component with multiple sources and an alt text.
@@ -23,12 +24,12 @@ export default function Picture({
   imgClassName?: string;
   style?: React.CSSProperties;
 }) {
-  // The blur logic is derived from https://github.com/vercel/next.js/blob/98be3ba23ea65ac5b581999d79a1093f147b46f0/packages/next/src/shared/lib/get-img-props.ts#L626
+  // The blur logic is derived from https://github.com/vercel/next.js/blob/98be3ba23ea65ac5b581999d79a1093f147b46f0/packages/next/src/shared/lib/get-img-props.ts
 
   // blurDataURL is a base64-encoded string generated by the nextjs image loader
-  // This is only generated during production builds, so in development, we might see
-  // no placeholders.
-  const blurDataURL = image.jpg.blurDataURL || "";
+  // This is only generated during production builds, and is set to a (broken) URL starting with "/_next"
+  // during development. In that case, we use a placeholder SVG instead.
+  const blurDataURL = (image.jpg.blurDataURL || "/_next").startsWith("/_next") ? `data:image/svg+xml;base64,${toBase64(shimmer(image.jpg.width, image.jpg.height))}` : image.jpg.blurDataURL as string;
 
   const backgroundImage = `url("data:image/svg+xml;charset=utf-8,${getImageBlurSvg(
     {

lib/utils.ts

@@ -383,4 +383,25 @@ export function maxBy<T>(array: T[], iteratee: (element: T) => number): T | unde
 
 export function minBy<T>(array: T[], iteratee: (element: T) => number): T | undefined {
   return maxBy(array, (element) => -iteratee(element));
-}
+}
+
+// Derived from https://github.com/vercel/next.js/blob/4380d0b79d07637abd50a584b51317eae1297885/examples/image-legacy-component/pages/shimmer.tsx#L4-L16
+export const shimmer = (w: number, h: number) => `
+<svg width="${w}" height="${h}" version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+  <defs>
+    <linearGradient id="g">
+      <stop stop-color="#333" offset="20%" />
+      <stop stop-color="#222" offset="50%" />
+      <stop stop-color="#333" offset="70%" />
+    </linearGradient>
+  </defs>
+  <rect width="${w}" height="${h}" fill="#333" />
+  <rect id="r" width="${w}" height="${h}" fill="url(#g)" />
+  <animate xlink:href="#r" attributeName="x" from="-${w}" to="${w}" dur="1s" repeatCount="indefinite"  />
+</svg>`;
+
+// Derived from https://github.com/vercel/next.js/blob/4380d0b79d07637abd50a584b51317eae1297885/examples/image-legacy-component/pages/shimmer.tsx#L18-L21
+export const toBase64 = (str: string) =>
+  typeof window === "undefined"
+    ? Buffer.from(str).toString("base64")
+    : window.btoa(str);