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

frontend/src/utils/encoding/bytesToBase64Url.spec.ts

@@ -0,0 +1,36 @@
+import { describe, expect, it } from 'vitest';
+import { base64UrlToBytes } from '@/utils/encoding/base64UrlToBytes';
+import { bytesToBase64Url } from '@/utils/encoding/bytesToBase64Url';
+
+const BASE64URL_PATTERN = /^[A-Za-z0-9_-]*$/;
+
+describe('bytesToBase64Url', () => {
+    it('uses only URL-safe characters and omits padding', () => {
+        const bytes = new Uint8Array([255, 254, 253, 252, 251, 250]);
+
+        const encoded = bytesToBase64Url(bytes);
+
+        expect(BASE64URL_PATTERN.test(encoded)).toBe(true);
+        expect(encoded.includes('+')).toBe(false);
+        expect(encoded.includes('/')).toBe(false);
+        expect(encoded.includes('=')).toBe(false);
+    });
+
+    it('roundtrips with base64UrlToBytes', () => {
+        for (const length of [0, 1, 2, 3, 4, 12, 16, 32, 64]) {
+            const bytes = new Uint8Array(length);
+
+            crypto.getRandomValues(bytes);
+
+            expect(base64UrlToBytes(bytesToBase64Url(bytes))).toEqual(bytes);
+        }
+    });
+
+    it('differs from standard base64 when URL encoding is required', () => {
+        const bytes = new Uint8Array([251, 255, 255]);
+
+        expect(bytesToBase64Url(bytes)).not.toBe(
+            btoa(Array.from(bytes, b => String.fromCharCode(b)).join(''))
+        );
+    });
+});