fix(mail): show child counts for collapsed folders

[keilogic]

Fixes #617.

Summary

• Track each folder's descendant folder paths when the folder tree is rebuilt.
• Show collapsed parent folders with the direct folder count plus counts from hidden child folders.
• Keep expanded parent folders showing their direct counts only, since the child rows are visible.
• Preserve the existing N/A display when a folder has no message-count entry yet.
• Add a focused folder-list component test for collapsed and expanded count display.

Validation

• npm ci
• 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
  • Passed with existing unused-variable warnings in these files.
• npx ng test --watch=false --browsers=FirefoxHeadless --include=src/app/folder/folderlist.component.spec.ts
  • TOTAL: 3 SUCCESS
• npm run lint -- --quiet
• npx ng test --watch=false --browsers=FirefoxHeadless
  • TOTAL: 246 SUCCESS, 2 skipped
• node src/build/pre-build.js
• npx ng build --configuration production --base-href=/app/ runbox7
  • Build hash: b64d515fe3fb2933
• node src/build/post-build.js
• npm run policy
  • Passed before and after commit. Output includes existing historical upstream commit-message warnings.
• git diff --check
• git show --stat --oneline --check HEAD

AI Disclosure

I used OpenAI Codex (GPT-5) in this local repository/worktree to inspect the folder list implementation, implement this patch, add the focused unit test, and run the validation commands listed above.