Add a new configuration option `list_all_free_models` to enable fetching and displaying free models from OpenRouter. Enhance model loading functions to include free models when enabled, and implement fallback logic in prompt generation to try alternative models if the primary one fails. Update the UI to display free models in a separate optgroup.
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.
