Fix sticky Square SDK rejection and clean up script element on failure

A load that resolved the script tag but failed to expose window.Square (or timed out) permanently cached a rejected promise, bricking card entry until reload. sdkPromise now resets and the injected script element is removed on every failure path so later calls retry fresh.

frontend/src/lib/square/square.ts

@@ -70,16 +70,28 @@ export function loadSquareSdk(): Promise<unknown> {
 		script.src = sdkUrl();
 		script.async = true;
 		script.dataset.squareSdk = 'true';
+
+		// Non-sticky-rejection invariant: a FAILED load attempt must never poison
+		// the cached promise. Concurrent callers during one attempt share the same
+		// in-flight promise (good — single script load), but a REJECTED promise is
+		// cleared here so the next getSquarePayments() call starts a fresh attempt
+		// instead of returning the same error forever. The dead <script> is also
+		// detached so a retry injects a clean element.
+		const fail = (error: Error) => {
+			script.remove();
+			sdkPromise = null;
+			reject(error);
+		};
+
 		script.onload = () => {
 			if (win.Square) {
 				resolve(win.Square);
 			} else {
-				reject(new Error('Square.js loaded but the Square global is missing'));
+				fail(new Error('Square.js loaded but the Square global is missing'));
 			}
 		};
 		script.onerror = () => {
-			sdkPromise = null;
+			fail(new Error('Failed to load Square Web Payments SDK'));
-			reject(new Error('Failed to load Square Web Payments SDK'));
 		};
 		document.head.appendChild(script);
 	});