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"]
+  }
+}