Website improvements

This revision preserves the AcademicPages/Jekyll structure and makes focused changes:

• clearer home-page research statement and direct links;
• a visible student-opening notice;
• compact research-focus and news layouts;
• updated research and teaching pages;
• improved typography, spacing, links, cards, and mobile behavior through _sass/_custom.scss;
• expanded site description for search results and link previews.

The site can still be deployed through the existing GitHub Pages workflow. No new Jekyll plugin is required.