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>

package.json

@@ -0,0 +1,46 @@
+{
+  "name": "@binect/js",
+  "version": "0.1.0",
+  "description": "JavaScript/TypeScript wrapper for the Binect API - Send PDF documents as physical mail",
+  "type": "module",
+  "main": "./dist/index.js",
+  "module": "./dist/index.js",
+  "types": "./dist/index.d.ts",
+  "exports": {
+    ".": {
+      "import": "./dist/index.js",
+      "types": "./dist/index.d.ts"
+    }
+  },
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "tsc",
+    "test": "node --experimental-vm-modules node_modules/vitest/vitest.mjs run",
+    "test:watch": "node --experimental-vm-modules node_modules/vitest/vitest.mjs",
+    "test:e2e": "node --experimental-vm-modules node_modules/vitest/vitest.mjs run tests/e2e.test.ts",
+    "typecheck": "tsc --noEmit",
+    "clean": "rm -rf dist",
+    "prepublishOnly": "npm run build"
+  },
+  "keywords": [
+    "binect",
+    "mail",
+    "letter",
+    "post",
+    "pdf",
+    "api",
+    "deutsche-post"
+  ],
+  "author": "",
+  "license": "MIT",
+  "devDependencies": {
+    "@types/node": "^20.10.0",
+    "typescript": "^5.3.0",
+    "vitest": "^1.0.0"
+  },
+  "engines": {
+    "node": ">=18.0.0"
+  }
+}