Add Binect SDK implementation, Explorer, and test suite

SDK (@binect/js):
- BinectClient with domain sub-clients (documents, sendings, accounts,
  attachments, invoices)
- HTTP Basic Auth, native fetch only (no runtime dependencies)
- TypeScript types matching Binect API vocabulary
- Status predicates and polling helpers in helpers.ts
- Structured error handling (BinectApiError, BinectAuthError)

Explorer:
- Standalone browser-based API explorer (explorer/index.html)
- Interactive testing without code

Tests:
- Unit tests for client, types, errors, helpers, http
- E2E tests for upload/delete and send/cancel workflows

Also includes:
- Architecture Decision Records (ADRs)
- Example DIN 5008 letter PDFs for testing
- API specification research notes

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

tests/client.test.ts

@@ -0,0 +1,59 @@
+import { describe, it, expect } from 'vitest';
+import { BinectClient } from '../src/client.js';
+import { DocumentsClient } from '../src/clients/documents.js';
+import { AttachmentsClient } from '../src/clients/attachments.js';
+import { SendingsClient } from '../src/clients/sendings.js';
+import { AccountsClient } from '../src/clients/accounts.js';
+import { InvoicesClient } from '../src/clients/invoices.js';
+
+describe('BinectClient', () => {
+  it('creates client with required config', () => {
+    const client = new BinectClient({
+      username: 'test@example.com',
+      password: 'password123',
+    });
+
+    expect(client).toBeInstanceOf(BinectClient);
+  });
+
+  it('creates client with custom baseUrl', () => {
+    const client = new BinectClient({
+      username: 'test@example.com',
+      password: 'password123',
+      baseUrl: 'https://custom.api.com/v1',
+    });
+
+    expect(client).toBeInstanceOf(BinectClient);
+  });
+
+  describe('sub-clients', () => {
+    let client: BinectClient;
+
+    beforeAll(() => {
+      client = new BinectClient({
+        username: 'test@example.com',
+        password: 'password123',
+      });
+    });
+
+    it('has documents client', () => {
+      expect(client.documents).toBeInstanceOf(DocumentsClient);
+    });
+
+    it('has attachments client', () => {
+      expect(client.attachments).toBeInstanceOf(AttachmentsClient);
+    });
+
+    it('has sendings client', () => {
+      expect(client.sendings).toBeInstanceOf(SendingsClient);
+    });
+
+    it('has accounts client', () => {
+      expect(client.accounts).toBeInstanceOf(AccountsClient);
+    });
+
+    it('has invoices client', () => {
+      expect(client.invoices).toBeInstanceOf(InvoicesClient);
+    });
+  });
+});