Fix base64 encoding for browser environment

The bufferToBase64 function from @binect/js expects Node.js Buffer
objects but was receiving browser ArrayBuffer, causing "[object ArrayBuffer]"
to be sent instead of valid base64. Use browser-native btoa() instead.

Also updates tests to work with @binect/js integration.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

jest.config.js

@@ -9,6 +9,9 @@ module.exports = {
     'src/**/*.{ts,tsx}',
     '!src/**/*.d.ts'
   ],
+  moduleNameMapper: {
+    '^@binect/js$': '<rootDir>/tests/__mocks__/@binect/js.ts'
+  },
   globals: {
     'ts-jest': {
       tsconfig: 'tsconfig.test.json'