feat: use blurHash when loading list backgrounds (#1188)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/frontend/pulls/1188

src/helpers/getBlobFromBlurHash.ts

@@ -0,0 +1,31 @@
+import {decode} from 'blurhash'
+
+export async function getBlobFromBlurHash(blurHash: string): Promise<Blob | null> {
+	if (blurHash === '') {
+		return null
+	}
+
+	const pixels = decode(blurHash, 32, 32)
+	const canvas = document.createElement('canvas')
+	canvas.width = 32
+	canvas.height = 32
+	const ctx = canvas.getContext('2d')
+	if (ctx === null) {
+		return null
+	}
+	
+	const imageData = ctx.createImageData(32, 32)
+	imageData.data.set(pixels)
+	ctx.putImageData(imageData, 0, 0)
+
+	return new Promise<Blob>((resolve, reject) => {
+		canvas.toBlob(b => {
+			if (b === null) {
+				reject(b)
+				return
+			}
+
+			resolve(b)
+		})
+	})
+}