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>

src/errors.ts

@@ -0,0 +1,71 @@
+import type { ApiErrorResponse } from './types.js';
+
+/**
+ * Error thrown when the Binect API returns a non-success response.
+ * Preserves HTTP status, endpoint, and parsed response body.
+ */
+export class BinectApiError extends Error {
+  /** HTTP status code */
+  public readonly status: number;
+  /** API endpoint that was called */
+  public readonly endpoint: string;
+  /** HTTP method used */
+  public readonly method: string;
+  /** Parsed error response from API (when available) */
+  public readonly response: ApiErrorResponse | null;
+
+  constructor(
+    message: string,
+    status: number,
+    endpoint: string,
+    method: string,
+    response: ApiErrorResponse | null = null
+  ) {
+    super(message);
+    this.name = 'BinectApiError';
+    this.status = status;
+    this.endpoint = endpoint;
+    this.method = method;
+    this.response = response;
+
+    // Maintains proper stack trace for where error was thrown (V8 engines)
+    if (Error.captureStackTrace) {
+      Error.captureStackTrace(this, BinectApiError);
+    }
+  }
+
+  /**
+   * Returns a detailed string representation of the error
+   */
+  toDetailedString(): string {
+    const parts = [
+      `BinectApiError: ${this.message}`,
+      `  Status: ${this.status}`,
+      `  Endpoint: ${this.method} ${this.endpoint}`,
+    ];
+
+    if (this.response) {
+      if (this.response.error) {
+        parts.push(`  Error: ${this.response.error}`);
+      }
+      if (this.response.message) {
+        parts.push(`  Message: ${this.response.message}`);
+      }
+      if (this.response.details && this.response.details.length > 0) {
+        parts.push(`  Details: ${this.response.details.join(', ')}`);
+      }
+    }
+
+    return parts.join('\n');
+  }
+}
+
+/**
+ * Error thrown when credentials are invalid or missing
+ */
+export class BinectAuthError extends BinectApiError {
+  constructor(endpoint: string, method: string, response: ApiErrorResponse | null = null) {
+    super('Authentication failed: Invalid credentials', 401, endpoint, method, response);
+    this.name = 'BinectAuthError';
+  }
+}