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>

tests/__mocks__/@binect/js.ts

@@ -0,0 +1,82 @@
+/**
+ * Mock for @binect/js library
+ */
+
+export class BinectApiError extends Error {
+  status: number;
+  response?: unknown;
+
+  constructor(message: string, status: number, response?: unknown) {
+    super(message);
+    this.name = 'BinectApiError';
+    this.status = status;
+    this.response = response;
+  }
+}
+
+export class BinectAuthError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'BinectAuthError';
+  }
+}
+
+export const EnvelopeType = {
+  DINLANG: 'DINLANG',
+  C4: 'C4',
+} as const;
+
+export const FrankingType = {
+  UNSPECIFIED: 'UNSPECIFIED',
+  STANDARD_FRANKING: 'STANDARD_FRANKING',
+  DV_FRANKING: 'DV_FRANKING',
+} as const;
+
+// Mock document response
+const mockDocument = {
+  id: 123,
+  filename: 'test.pdf',
+  numberOfPages: 2,
+  status: { code: 2, text: 'shippable' },
+  documentType: 'LETTER',
+  letter: {
+    letterType: 'LetterData',
+    letterData: {
+      recipientAddress: 'Test Address',
+      price: { priceBeforeTax: 100, priceAfterTax: 119, unit: 'EUROCENT', taxInPercent: 19 },
+      international: false,
+      options: { simplex: false, color: false },
+    },
+  },
+};
+
+// Mock account response
+const mockAccount = {
+  id: 1,
+  email: 'test@example.com',
+};
+
+export class BinectClient {
+  documents = {
+    upload: jest.fn().mockResolvedValue(mockDocument),
+  };
+
+  accounts = {
+    get: jest.fn().mockResolvedValue(mockAccount),
+  };
+
+  constructor(_config: { username: string; password: string }) {
+    // Store config if needed for tests
+  }
+}
+
+export type Document = typeof mockDocument;
+
+export interface DocumentUploadOptions {
+  content: string;
+  filename: string;
+  simplex?: boolean;
+  color?: boolean;
+  envelope?: string;
+  franking?: string;
+}