Fix indentation styling inconsistency in file tree browser with extra <li/> wrapper

Problem

The file tree browser currently has an indentation styling issue due to an extra <li/> wrapper that was added when we moved away from recycleStroller for accessibility compliance. This creates visual inconsistencies where the indentation doesn't happen on the <li> element, making the hover state less clear about file hierarchy position

See !214276 (comment 2918560792)

Background Context

• file_tree component is used in two places:
  • Current file tree browser
  • Rapid diffs MR tree
• The extra <li/> wrapper exists because we removed recycleStroller for accessibility compliance (see !207844 (merged))
• We're still evaluating whether to bring back recycleStroller for performance benefits (#577630 (closed))
• Current indentation styling is controlled by complex SCSS selectors

Proposed Solutions

Option 1: If keeping standard scroller

• Override indentation styling in file_row to work properly with the <li/> wrapper
• Ensure new styling integrates well with existing indentation line SCSS
• Make indentation happen on the <li> element for clearer hierarchy visualization

Option 2: If bringing back recycleStroller

• Remove the <li/> wrapper (problem would be resolved automatically)
• Ensure accessibility compliance is maintained

Acceptance Criteria

• Indentation styling is consistent between file tree browser and rapid diffs MR tree
• Hover states clearly indicate file hierarchy position
• Solution works with existing SCSS indentation line selectors