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

frontend/src/utils/encoding/base64ToBytes.spec.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest';
+import { base64ToBytes } from '@/utils/encoding/base64ToBytes';
+
+describe('base64ToBytes', () => {
+    it('decodes RFC 4648 examples', () => {
+        expect(base64ToBytes('Zg==')).toEqual(new Uint8Array([102]));
+        expect(base64ToBytes('Zm8=')).toEqual(new Uint8Array([102, 111]));
+        expect(base64ToBytes('Zm9v')).toEqual(new Uint8Array([102, 111, 111]));
+        expect(base64ToBytes('Zm9vYg==')).toEqual(new Uint8Array([102, 111, 111, 98]));
+    });
+
+    it('decodes empty input', () => {
+        expect(base64ToBytes('')).toEqual(new Uint8Array([]));
+    });
+
+    it('decodes every byte value via roundtrip with btoa', () => {
+        const bytes = new Uint8Array(256);
+
+        for (let i = 0; i < 256; i++) {
+            bytes[i] = i;
+        }
+
+        const binary = Array.from(bytes, byte => String.fromCharCode(byte)).join('');
+        const base64 = btoa(binary);
+
+        expect(base64ToBytes(base64)).toEqual(bytes);
+    });
+
+    it('rejects invalid base64', () => {
+        expect(() => base64ToBytes('not!!!valid')).toThrow();
+    });
+});