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>

package.json

@@ -10,10 +10,10 @@
     "test:ui": "vitest --ui"
   },
   "devDependencies": {
-    "@vitest/ui": "^2.1.0",
-    "@vitest/coverage-v8": "^2.1.0",
+    "@vitest/coverage-v8": "^4.0.14",
+    "@vitest/ui": "^4.0.14",
     "jsdom": "^25.0.0",
-    "vitest": "^2.1.0"
+    "vitest": "^4.0.14"
   },
   "keywords": [
     "timeline",
@@ -24,4 +24,4 @@
   ],
   "author": "",
   "license": "MIT"
-}
+}