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>

architecture/ADR-002-no-external-dependencies.md

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+# ADR-002: No External Runtime Dependencies
+
+## Status
+Accepted
+
+## Context
+The SDK needs to make HTTP requests and handle authentication. Common approaches include using libraries like axios, node-fetch, or got for HTTP, and various utilities for base64 encoding.
+
+Per the TSD (Section 2, Design Guardrail #1): "No backend dependency - The product must function entirely in browser and JavaScript runtime environments."
+
+## Decision
+
+The SDK will have **zero runtime dependencies**:
+
+1. **HTTP Requests**: Use native `fetch` API
+   - Available in all modern browsers
+   - Built into Node.js >= 18
+   - No polyfills required for target environments
+
+2. **Base64 Encoding**: Use native APIs
+   - Browser: `btoa()` / `atob()`
+   - Node.js: `Buffer.from().toString('base64')`
+   - Provide isomorphic wrapper
+
+3. **Type Checking**: TypeScript (dev dependency only)
+
+## Consequences
+
+### Positive
+- No dependency vulnerabilities to manage
+- Smaller bundle size
+- Predictable behavior (no library-specific quirks)
+- Works identically in browser and Node.js
+- No version conflicts with consumer projects
+
+### Negative
+- Must implement utility functions ourselves
+- Cannot leverage library conveniences (interceptors, etc.)
+- Requires Node.js >= 18 (has native fetch)
+
+## Alternatives Considered
+
+1. **axios**: Popular but adds ~13KB and has had security vulnerabilities
+2. **node-fetch**: Would require different code paths for browser/Node
+3. **ky**: Modern but still an external dependency
+
+## References
+- TSD: Section 2 (Design Guardrails)
+- Node.js fetch: https://nodejs.org/docs/latest-v18.x/api/globals.html#fetch