feature/add collapsible/expandable folder tree with toggle controls

[raj-kapil]

This pull request introduces a collapsible folder feature to the file tree UI, allowing users to expand and collapse all folders or individual folders for improved navigation. The changes span HTML, JavaScript, and CSS to implement the new functionality and update the visual presentation.

Collapsible Tree Functionality

• Added "Expand All" and "Collapse All" buttons to the UI for bulk folder toggling (index.html).
• Implemented folder expand/collapse logic, including event handlers for row clicks, bulk toggling, and renumbering visible lines (script.js). [1] [2] [3]
• Updated the visual hierarchy builder to track folder depth and added caret icons for folders (script.js). [1] [2]
• Registered new DOM elements and event listeners for expand/collapse actions (script.js). [1] [2]

Styling Enhancements

• Added CSS rules for collapsible tree folders, caret icons, and hidden child rows (style.css).

[mgks]

Looks nice. Thanks for the contribution!