feat: Implement functional web browsing with HTTP + HTML rendering

Implemented the missing browser engine functionality to make TUT actually
browse the web.

Browser Engine Changes:
- Integrate HttpClient to fetch URLs via GET requests
- Integrate HtmlRenderer to parse and render HTML content
- Implement proper error handling for failed HTTP requests
- Add relative URL resolution for links (absolute and relative paths)
- Store title, content, and links from rendered pages

Tested with https://tldp.org/HOWTO/HOWTO-INDEX/howtos.html:
✅ Successfully fetches and displays web pages
✅ Renders HTML with proper formatting (headings, lists, links)
✅ Extracts and numbers clickable links
✅ Displays page titles

The browser is now fully functional for basic text-based web browsing!

src/core/browser_engine.cpp

@@ -4,6 +4,8 @@
  */
 
 #include "core/browser_engine.hpp"
+#include "core/http_client.hpp"
+#include "renderer/html_renderer.hpp"
 
 namespace tut {
 
@@ -15,6 +17,9 @@ public:
     std::vector<LinkInfo> links_;
     std::vector<std::string> history_;
     size_t history_index_{0};
+
+    HttpClient http_client_;
+    HtmlRenderer renderer_;
 };
 
 BrowserEngine::BrowserEngine() : impl_(std::make_unique<Impl>()) {}
@@ -22,14 +27,66 @@ BrowserEngine::BrowserEngine() : impl_(std::make_unique<Impl>()) {}
 BrowserEngine::~BrowserEngine() = default;
 
 bool BrowserEngine::loadUrl(const std::string& url) {
-    // TODO: 实现 HTTP 请求和 HTML 解析
     impl_->current_url_ = url;
-    return true;
+
+    // 发送 HTTP 请求
+    auto response = impl_->http_client_.get(url);
+
+    if (!response.isSuccess()) {
+        // 加载失败，设置错误内容
+        impl_->title_ = "Error";
+        impl_->content_ = "Failed to load page: " +
+                          (response.error.empty()
+                           ? "HTTP " + std::to_string(response.status_code)
+                           : response.error);
+        impl_->links_.clear();
+        return false;
+    }
+
+    // 渲染 HTML
+    return loadHtml(response.body);
 }
 
 bool BrowserEngine::loadHtml(const std::string& html) {
-    // TODO: 实现 HTML 解析
-    impl_->content_ = html;
+    // 渲染 HTML
+    RenderOptions options;
+    options.show_links = true;
+    options.use_colors = true;
+
+    auto result = impl_->renderer_.render(html, options);
+
+    impl_->title_ = result.title;
+    impl_->content_ = result.text;
+    impl_->links_ = result.links;
+
+    // 解析相对 URL
+    if (!impl_->current_url_.empty()) {
+        for (auto& link : impl_->links_) {
+            if (link.url.find("://") == std::string::npos) {
+                // 简单的相对 URL 解析
+                if (!link.url.empty() && link.url[0] == '/') {
+                    // 绝对路径
+                    size_t scheme_end = impl_->current_url_.find("://");
+                    if (scheme_end != std::string::npos) {
+                        size_t host_end = impl_->current_url_.find('/', scheme_end + 3);
+                        std::string base = (host_end != std::string::npos)
+                            ? impl_->current_url_.substr(0, host_end)
+                            : impl_->current_url_;
+                        link.url = base + link.url;
+                    }
+                } else if (link.url.find("://") == std::string::npos &&
+                           !link.url.empty() && link.url[0] != '#') {
+                    // 相对路径
+                    size_t last_slash = impl_->current_url_.rfind('/');
+                    if (last_slash != std::string::npos) {
+                        std::string base = impl_->current_url_.substr(0, last_slash + 1);
+                        link.url = base + link.url;
+                    }
+                }
+            }
+        }
+    }
+
     return true;
 }
 

src/core/browser_engine.hpp

@@ -11,18 +11,10 @@
 #include <memory>
 #include <optional>
 #include <vector>
+#include "types.hpp"
 
 namespace tut {
 
-/**
- * @brief 链接信息结构体
- */
-struct LinkInfo {
-    std::string url;    ///< 链接 URL
-    std::string text;   ///< 链接文本
-    int line{0};        ///< 所在行号
-};
-
 /**
  * @brief 浏览器引擎类
  *

src/renderer/html_renderer.hpp

@@ -10,11 +10,10 @@
 #include <string>
 #include <vector>
 #include <memory>
+#include "../core/types.hpp"
 
 namespace tut {
 
-struct LinkInfo;
-
 /**
  * @brief 渲染选项
  */

src/ui/content_view.hpp

@@ -11,11 +11,10 @@
 #include <vector>
 #include <functional>
 #include <memory>
+#include "../core/types.hpp"
 
 namespace tut {
 
-struct LinkInfo;
-
 /**
  * @brief 内容视图组件类
  *