fix(folder): place created folders after parent subtree

[keilogic]

Fixes #506.

Summary

• Add a shared folder-creation order helper that inserts the new-folder placeholder by matching the actual parent folder id.
• Append new root folders after the existing folder list, and insert new subfolders after the selected parent folder's current descendant subtree.
• Add focused FolderList coverage for root-folder creation, selected-folder child creation, and menu-selected subfolder creation.

Testing

• npm ci
• npx ng test runbox7 --watch=false --browsers=FirefoxHeadless --include src/app/folder/folderlist.component.spec.ts (4 success; existing folder move console logs)
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npx eslint src/app/folder/folderlist.component.ts src/app/folder/folderlist.component.spec.ts --quiet
• npm run lint -- --quiet
• git diff --check origin/master...HEAD
• git show --stat --oneline --check HEAD
• npm run policy (current commits OK; historical upstream commit-message warnings still printed)
• $env:SKIP_CHANGELOG='1'; node src/build/pre-build.js; npx ng build --configuration production --base-href=/app/ runbox7; node src/build/post-build.js (production build hash 686c1ba4c6401b75; existing Angular/Sass/CommonJS warnings)

AI Disclosure

I used OpenAI Codex/GPT-5 in a local development workspace to inspect the relevant code, implement this change, and run the validation commands listed above. I reviewed the resulting diff and validation output before submitting.