Add project documentation and configure for JavaScript/TypeScript

- Add PRD and TSD defining SDK and Explorer architecture
- Add CLAUDE.md with project guidance for Claude Code
- Update README with project description
- Replace Python .gitignore with JS/TS configuration

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+Binect-JS is a JavaScript/TypeScript wrapper for the Binect REST API (https://app.binect.de/index.jsp?id=api) that enables sending PDF documents as physical mail. The project consists of two artifacts:
+
+1. **Binect-JS SDK** (`@binect/js`) - A thin API wrapper
+2. **Binect Explorer** (`@binect/explorer`) - A browser-based interactive tool for learning and experimentation
+
+## Architecture Principles
+
+The SDK is organized around domain-aligned sub-clients that mirror the API vocabulary:
+- `documents` - PDF upload, status inspection, parameter modification
+- `attachments` - Attachment handling
+- `sendings` - Mail dispatch triggers, cancellation
+- `accounts` - Account management
+- `invoices` - Invoice access
+
+### SDK Layer Separation
+
+**Core API Layer** (authoritative): 1:1 semantic mapping to REST endpoints. Methods like `documents.uploadPdf`, `documents.getStatus`, `sendings.announce`, `sendings.cancel`.
+
+**Convenience Layer** (optional, non-authoritative): Additive helpers like status predicates (`isShippable`), error extraction, polling helpers. These must never be the only way to perform an action.
+
+## Design Constraints
+
+- **No backend dependency**: Must function entirely in browser/JS runtime
+- **No semantic reinterpretation**: Wrapper must not alter business meaning or outcomes
+- **Transparency over abstraction**: Developers must reason about actual API calls
+- **No default retries**: Network behavior must be explicit and opt-in
+- **Authentication**: HTTP Basic Auth, credentials are ephemeral (not stored/cached)
+
+## API Integration Notes
+
+- Uploads use base64-encoded PDFs
+- All non-success responses surface as structured errors preserving HTTP status, endpoint, and parsed response
+- The wrapper does not reinterpret business errors from the API