Introduce a `base.html` template to establish a common structure for all pages using Jinja2 template inheritance. This eliminates redundant HTML boilerplate across multiple files.
Additionally, common CSS rules have been extracted from individual templates and consolidated into a single `static/css/main.css` file. All pages now link to this shared stylesheet.
This refactoring significantly reduces code duplication, improves maintainability, and ensures a consistent user interface.
