fix(Folder): align file list entries and unify spacing in file explorer

[spider-yamet]

Summary

Fixes file explorer alignment and spacing so folder/file entries line up and spacing is consistent (resolves #1217).

Changes

Layout

• First column: File rows now use a fixed 16×16 box (same as folder chevron) instead of a width-only spacer, so icons and labels align vertically for both folders and files.
• Spacing: Added gap-2 on each row and removed mr-2 from icons so spacing between chevron/spacer, icon, and label is uniform (8px).
• Accessibility: File spacer has aria-hidden so it’s ignored by assistive tech.

Code

• src/components/Folder/index.tsx
  • File spacer: w-4 → flex h-4 w-4 flex-shrink-0 items-center justify-center + aria-hidden.
  • Folder chevron wrapper: added flex-shrink-0 for consistent first-column width.
  • Row button: added gap-2, removed mr-2 from FolderIcon, file icon, and FileText.
  • Exported FileTree for unit tests.

Tests

• test/unit/components/Folder/FileTree.test.tsx (new)
  • Renders folder and file rows.
  • First column uses consistent h-4/w-4 for alignment.
  • Row uses gap-2 for spacing.
  • File spacer has aria-hidden.
  • onToggleFolder / onSelectFile on click.
  • Returns null when node has no children.

Testing

• npm run lint — pass
• npm run test -- --run test/unit/components/Folder/FileTree.test.tsx — 7 tests pass

##Resolves

• [Feature Request] refactor file list alignment and layout inconsistencies #1217

[spider-yamet]

@Wendong-Fan @Pakchoioioi @bytecii Could you please review my PR?

Regards

[Pakchoioioi]

Functional requirements met. Ready for code review.

[bytecii]

LGTM