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/errors.test.ts

@@ -0,0 +1,89 @@
+import { describe, it, expect } from 'vitest';
+import { BinectApiError, BinectAuthError } from '../src/errors.js';
+
+describe('BinectApiError', () => {
+  it('creates error with all properties', () => {
+    const response = { error: 'Not found', message: 'Document not found' };
+    const error = new BinectApiError('Not found', 404, '/documents/123', 'GET', response);
+
+    expect(error.name).toBe('BinectApiError');
+    expect(error.message).toBe('Not found');
+    expect(error.status).toBe(404);
+    expect(error.endpoint).toBe('/documents/123');
+    expect(error.method).toBe('GET');
+    expect(error.response).toEqual(response);
+  });
+
+  it('creates error without response', () => {
+    const error = new BinectApiError('Server error', 500, '/documents', 'POST');
+
+    expect(error.status).toBe(500);
+    expect(error.response).toBeNull();
+  });
+
+  it('has correct inheritance', () => {
+    const error = new BinectApiError('Test', 400, '/test', 'GET');
+
+    expect(error instanceof Error).toBe(true);
+    expect(error instanceof BinectApiError).toBe(true);
+  });
+
+  describe('toDetailedString', () => {
+    it('includes all error details', () => {
+      const response = {
+        error: 'Validation error',
+        message: 'Invalid document format',
+        details: ['Page size incorrect', 'Missing address'],
+      };
+      const error = new BinectApiError('Validation failed', 400, '/documents', 'POST', response);
+
+      const detailed = error.toDetailedString();
+
+      expect(detailed).toContain('BinectApiError: Validation failed');
+      expect(detailed).toContain('Status: 400');
+      expect(detailed).toContain('Endpoint: POST /documents');
+      expect(detailed).toContain('Error: Validation error');
+      expect(detailed).toContain('Message: Invalid document format');
+      expect(detailed).toContain('Details: Page size incorrect, Missing address');
+    });
+
+    it('handles minimal error response', () => {
+      const error = new BinectApiError('Server error', 500, '/test', 'GET');
+
+      const detailed = error.toDetailedString();
+
+      expect(detailed).toContain('BinectApiError: Server error');
+      expect(detailed).toContain('Status: 500');
+      // Should not contain "Error:" as a separate field (only in class name)
+      expect(detailed).not.toContain('  Error:');
+      expect(detailed).not.toContain('  Message:');
+    });
+  });
+});
+
+describe('BinectAuthError', () => {
+  it('creates authentication error', () => {
+    const error = new BinectAuthError('/accounts', 'GET');
+
+    expect(error.name).toBe('BinectAuthError');
+    expect(error.message).toBe('Authentication failed: Invalid credentials');
+    expect(error.status).toBe(401);
+    expect(error.endpoint).toBe('/accounts');
+    expect(error.method).toBe('GET');
+  });
+
+  it('includes response when provided', () => {
+    const response = { error: 'Unauthorized', message: 'Invalid credentials' };
+    const error = new BinectAuthError('/accounts', 'GET', response);
+
+    expect(error.response).toEqual(response);
+  });
+
+  it('inherits from BinectApiError', () => {
+    const error = new BinectAuthError('/test', 'GET');
+
+    expect(error instanceof Error).toBe(true);
+    expect(error instanceof BinectApiError).toBe(true);
+    expect(error instanceof BinectAuthError).toBe(true);
+  });
+});