add frontend AES-GCM and base64 helpers with Vitest coverage.

frontend/src/utils/encoding/base64UrlToBytes.ts

@@ -0,0 +1,13 @@
+import { base64ToBytes } from '@/utils/encoding/base64ToBytes';
+
+export const base64UrlToBytes = (base64Url: string): Uint8Array => {
+    let base64 = base64Url.split('-').join('+').split('_').join('/');
+
+    const padding = base64.length % 4;
+
+    if (padding > 0) {
+        base64 += '='.repeat(4 - padding);
+    }
+
+    return base64ToBytes(base64);
+};