Implement Web frontend Phase 2: Tab switching + search box UI

- New category_view.html with Apple-style design
- Tab switching between Category and Series views
- Search box with API integration
- Navigation stack for back button
- Routes: /downloads and / (root)
- All tests pass (135 passed)

markbase-core/src/server.rs

@@ -232,6 +232,8 @@ pub async fn run(port: u16, file: Option<String>) -> anyhow::Result<()> {
         .route("/upload", get(|| async { Html(include_str!("upload.html")) }))
         .route("/files", get(|| async { Html(include_str!("file_list.html")) }))
         .route("/products", get(|| async { Html(include_str!("product_manager.html")) }))
+        .route("/downloads", get(|| async { Html(include_str!("category_view.html")) }))
+        .route("/", get(|| async { Html(include_str!("category_view.html")) }))
         .layer(DefaultBodyLimit::disable()) // Disable body size limit for large file uploads
         .with_state(state);