Fix intermittently failing Playwright tests in main (#6541)

# Description of Changes
[#6474](https://github.com/Stirling-Tools/Stirling-PDF/pull/6474)
updated the IndexedDB schema number to v9, but a couple of Playwright
tests were explicitly creating a DB in v4 schema, which then caused
inconsistently failing tests because the DB upgrade process is
asynchronous and sometimes was too slow to upgrade, causing the test to
get into an invalid state.

Also fixes the screenshots directory exclusion since the frontend folder
was restructured.

.gitignore

@@ -279,4 +279,4 @@ docs/type3/signatures/
 *.playwright-mcp.png
 
 # Local screenshot artifacts from *-screenshots.spec.ts
-frontend/screenshots/
+frontend/editor/screenshots/