test: improve test infrastructure and fix test assertions

- Add mockPapaParse helper to centralize CSV mocking
- Fix test assertions to match actual output (German month names)
- Add missing DOM elements to test setup (projectFile, csvFile, etc.)
- Update vitest to v4.0.14 for improved testing capabilities
- Make setupEventHandlers globally accessible for testing
- Use textContent instead of innerText for better consistency
- Fix autoLoadDefaultProject test to check all three fallback paths
- Add proper event handler setup in integration tests
- Improve error handling test assertions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

test/setup.js

@@ -35,5 +35,9 @@ export const setupBasicDOM = () => {
     <div id="viewer"></div>
     <link id="dynamicCss" rel="stylesheet" href="">
     <button id="downloadSvg"></button>
+    <span id="projectFile"></span>
+    <span id="csvFile"></span>
+    <span id="svgFile"></span>
+    <span id="cssFile"></span>
   `
 }