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>

.eslintrc.json

@@ -0,0 +1,22 @@
+{
+  "root": true,
+  "parser": "@typescript-eslint/parser",
+  "parserOptions": {
+    "ecmaVersion": 2022,
+    "sourceType": "module",
+    "project": "./tsconfig.json"
+  },
+  "plugins": ["@typescript-eslint"],
+  "extends": [
+    "eslint:recommended",
+    "plugin:@typescript-eslint/recommended",
+    "plugin:@typescript-eslint/recommended-requiring-type-checking"
+  ],
+  "rules": {
+    "@typescript-eslint/explicit-function-return-type": "warn",
+    "@typescript-eslint/no-unused-vars": ["error", { "argsIgnorePattern": "^_" }],
+    "@typescript-eslint/no-explicit-any": "error",
+    "no-console": "warn"
+  },
+  "ignorePatterns": ["dist", "node_modules", "*.js", "vitest.config.ts"]
+}