test(e2e): improve dialog focus tests and code quality

Test improvements:
- Add error handling for Electron app launch in headless environments
- Update dialog selectors to use ARIA attributes for better reliability
- Implement actual test logic (previously skipped placeholders)
- Add screenshot capture evidence for test results
- Update test descriptions to match actual dialog types

Code quality improvements:
- Change 'let' to 'const' for variables that are not reassigned
- Improves code clarity and follows best practices

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/renderer/src/hooks/useCleaner.ts

@@ -297,7 +297,7 @@ export function useCleaner() {
 
     try {
       setIsRunning(true)
-      let msgParts: string[] = []
+      const msgParts: string[] = []
 
       if (materialsToUpsert.length > 0) {
         const res = await window.electron.materials.upsertBatch(materialsToUpsert)