perf(sprites): serve grid-sized thumbnails from an immutable URL space

Every grid cell downloaded the full detail sprite. Generate a smaller
/sprites-grid/v1/ set at build time and let the grid ask for those first,
falling back through the existing detail URLs when a thumbnail is missing so
detail resolution is unchanged. The new prefix is versioned, so it can be cached
forever, and the service worker recognises it alongside the other sprite roots.

The placeholder is now an empty box rather than a spinner: a thousand spinners
cost layout work and announced nothing useful.

scripts/grid-thumbnails.mjs

@@ -0,0 +1,35 @@
+import { readdir, mkdir, stat, writeFile } from 'node:fs/promises';
+import path from 'node:path';
+import sharp from 'sharp';
+
+// Bump the URL version whenever dimensions, quality or source artwork changes.
+const source = 'static/sprites-small/home';
+const destination = 'static/sprites-grid/v1/home';
+const files = [];
+async function walk(relative = '') {
+	for (const entry of await readdir(path.join(source, relative), { withFileTypes: true })) {
+		const name = path.join(relative, entry.name);
+		if (entry.isDirectory()) await walk(name);
+		else if (entry.name.endsWith('.webp')) files.push(name);
+	}
+}
+await walk();
+const manifest = [];
+for (const relative of files.sort()) {
+	const target = path.join(destination, relative);
+	await mkdir(path.dirname(target), { recursive: true });
+	try {
+		await stat(target);
+	} catch {
+		await sharp(path.join(source, relative))
+			.resize(128, 128, { fit: 'inside', withoutEnlargement: true })
+			.webp({ quality: 80 })
+			.toFile(target);
+	}
+	manifest.push({ path: relative.split(path.sep).join('/'), bytes: (await stat(target)).size });
+}
+await writeFile(
+	'static/sprites-grid/v1/manifest.json',
+	JSON.stringify({ version: 1, width: 128, quality: 80, files: manifest })
+);
+console.log(`Prepared ${files.length} versioned grid thumbnails; originals preserved.`);