Fix categories filter

[Kuehnelt]

I noticed that the categories filter was hardcoded (see code below) to only include 'science,' 'tech,' and 'news,' ignoring any user-created categories. I have changed the logic to use the categories created by the user instead. I also had to make some changes to related functions to ensure everything loads correctly.

                KeyCode::Char('c') => {
                    // Toggle category filter
                    if app.filter_options.category.is_none() {
                        // Cycle through available categories (tech, news, etc.)
                        app.filter_options.category = Some("tech".to_string());
                    } else if app.filter_options.category.as_deref() == Some("tech") {
                        app.filter_options.category = Some("news".to_string());
                    } else if app.filter_options.category.as_deref() == Some("news") {
                        app.filter_options.category = Some("science".to_string());
                    } else {
                        app.filter_options.category = None;
                    }
                    app.apply_filters();
                }

[bahdotsh]

@Kuehnelt please format the code using cargo fmt to fix the issues!