chore: robustify playground decode (#17606)

For some reason LLMs like to percent-encode stuff, this makes the download script still run in that case

playgrounds/sandbox/scripts/download.js

@@ -635,7 +635,19 @@ if (is_local) {
 } else if (url && url.origin === 'https://svelte.dev' && url.pathname.startsWith('/playground/')) {
 	// Svelte playground URL handling (existing logic)
 	if (url.hash.length > 1) {
-		const decoded = atob(url.hash.slice(1).replaceAll('-', '+').replaceAll('_', '/'));
+		// Decode percent-encoded characters (e.g., %5F => _), and replace base64 chars.
+		let decoded;
+		try {
+			// First, decode URI components to handle %xx encodings (e.g. %5F -> _) (LLMs calling this script sometimes encode them for some reason)
+			decoded = url.hash.slice(1);
+			decoded = decodeURIComponent(decoded);
+
+			// Now, restore for base64 (replace -/+, _/ /)
+			decoded = atob(decoded.replaceAll('-', '+').replaceAll('_', '/'));
+		} catch (e) {
+			console.error('Failed to decode URL hash:', e);
+			process.exit(1);
+		}
 		// putting it directly into the blob gives a corrupted file
 		const u8 = new Uint8Array(decoded.length);
 		for (let i = 0; i < decoded.length; i++) {