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/testHelpers.js

@@ -52,11 +52,30 @@ export const createSampleTemplate = () => `<svg xmlns="http://www.w3.org/2000/sv
 export const mockFetch = (data, ok = true) => {
   global.fetch.mockResolvedValueOnce({
     ok,
+    status: ok ? 200 : 404,
+    statusText: ok ? 'OK' : 'Not Found',
     json: () => Promise.resolve(data),
     text: () => Promise.resolve(typeof data === 'string' ? data : JSON.stringify(data))
   })
 }
 
+// Helper to mock Papa.parse with proper CSV parsing
+export const mockPapaParse = () => {
+  global.Papa.parse.mockImplementation((text, options) => {
+    const lines = text.trim().split('\n')
+    const headers = lines[0].split(',')
+    const data = lines.slice(1).map(line => {
+      const values = line.split(',')
+      const obj = {}
+      headers.forEach((header, i) => {
+        obj[header] = values[i]
+      })
+      return obj
+    })
+    options.complete({ data })
+  })
+}
+
 export const expectElementToHaveText = (selector, text) => {
   const element = document.querySelector(selector)
   expect(element).toBeTruthy()