Add comprehensive mobile-first responsive styles across billing, code-editor, file-upload, and main style sheets. Introduce a dedicated mobile.css file (702 lines) with hamburger navigation, sidebar overlay, pull-to-refresh indicator, and touch-friendly sizing. Integrate GestureHandler and PullToRefreshIndicator classes into file-list, file-preview, and file-upload-view components for swipe-to-close and pull-to-refresh interactions. Remove legacy inline media queries from style.css and link mobile.css in index.html.
