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.json

@@ -0,0 +1,29 @@
+{
+  "name": "e2e-test-data",
+  "version": "1.0.0",
+  "description": "Test JSON file for E2E testing",
+  "metadata": {
+    "created": "2024-01-01T00:00:00Z",
+    "author": "E2E Test Suite"
+  },
+  "data": {
+    "items": [
+      { "id": 1, "name": "Item One", "value": 100 },
+      { "id": 2, "name": "Item Two", "value": 200 },
+      { "id": 3, "name": "Item Three", "value": 300 }
+    ],
+    "nested": {
+      "level1": {
+        "level2": {
+          "key": "deep-value"
+        }
+      }
+    },
+    "tags": ["test", "e2e", "conversion"]
+  },
+  "settings": {
+    "enabled": true,
+    "threshold": 0.75,
+    "options": ["a", "b", "c"]
+  }
+}

tests/e2e/fixtures/test.md

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+# 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.*

tests/e2e/fixtures/test.svg

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+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="200" height="200" viewBox="0 0 200 200">
+  <!-- Background -->
+  <rect width="200" height="200" fill="#f0f0f0"/>
+  
+  <!-- Main shape -->
+  <rect x="20" y="20" width="160" height="160" fill="#4285f4" rx="20"/>
+  
+  <!-- Inner circle -->
+  <circle cx="100" cy="100" r="50" fill="#ffffff"/>
+  
+  <!-- Cross lines -->
+  <line x1="100" y1="60" x2="100" y2="140" stroke="#4285f4" stroke-width="8"/>
+  <line x1="60" y1="100" x2="140" y2="100" stroke="#4285f4" stroke-width="8"/>
+  
+  <!-- Text label -->
+  <text x="100" y="180" text-anchor="middle" font-family="Arial, sans-serif" font-size="14" fill="#333">E2E Test</text>
+</svg>