test(e2e): add End-to-End test suite

Added comprehensive E2E tests:

1. API E2E Tests (api.e2e.test.ts):
   - Healthcheck API testing
   - Static resources verification
   - Module loading tests
   - Converter properties validation
   - File type normalization tests
   - i18n module tests
   - Transfer module tests
   - Database structure tests

2. Converter E2E Tests (converters.e2e.test.ts):
   - Real Inkscape conversions (SVG → PNG/PDF/EPS)
   - Real Pandoc conversions (Markdown → HTML/DOCX/RST)
   - Dasel format conversions (JSON → YAML/TOML)
   - Batch conversion tests
   - Error handling tests
   - Auto-detection of available tools

3. Test Infrastructure:
   - helpers.ts: Tool detection, test file generation
   - fixtures/: Sample test files (SVG, MD, JSON)
   - .gitignore: Exclude output directory

Total: 34 new E2E tests

tests/e2e/fixtures/test.md

@@ -0,0 +1,46 @@
+# E2E Test Document
+
+This is a test document for end-to-end testing of the ConvertX file conversion system.
+
+## Introduction
+
+ConvertX is a self-hosted file conversion tool that supports multiple formats.
+
+## Features
+
+- **Multiple Formats**: Support for images, documents, audio, video, and more
+- **Self-Hosted**: Complete control over your data
+- **i18n Support**: Available in 65+ languages
+- **Chunk Transfer**: Efficient handling of large files
+
+## Code Example
+
+```python
+def hello_world():
+    print("Hello, ConvertX!")
+    return True
+```
+
+## Table Example
+
+| Format | Type | Converter |
+|--------|------|-----------|
+| SVG | Image | Inkscape |
+| PDF | Document | LibreOffice |
+| MP4 | Video | FFmpeg |
+| DOCX | Document | Pandoc |
+
+## List Example
+
+1. Upload your file
+2. Select the target format
+3. Click convert
+4. Download the result
+
+## Conclusion
+
+Thank you for using ConvertX!
+
+---
+
+*This document is used for E2E testing purposes.*