feat(travel): Implement settings UI and web server for MTR route configuration

- Added MainUIHandler class to manage the main UI and polling for arrival data.
- Introduced SettingsUI class for displaying QR code and configuration options.
- Created SettingsUIHandler to manage settings UI lifecycle and web server interactions.
- Developed WebHandler to handle HTTP requests for MTR route settings, including adding and removing routes.
- Implemented web endpoints for fetching MTR lines, routes, and saving settings.
- Enhanced UI with responsive design for e-ink displays and added error handling for web interactions.

main/ui/apps/travel/ui/main_handler.h

@@ -0,0 +1,69 @@
+#pragma once
+
+#include "ui/apps/travel/ui/main.h"
+#include "ui/apps/travel/settings/settings_handler.h"
+#include "ui/interaction_handler.h"
+#include "external/mtr/mtr.h"
+#include "network/network.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "freertos/semphr.h"
+#include "esp_err.h"
+#include <memory>
+#include <vector>
+#include <atomic>
+
+namespace travel {
+
+/**
+ * @brief Main UI Handler for Travel app
+ *
+ * Manages the MainUI instance, polling task, and MTR API interactions.
+ * Runs a background task to periodically fetch arrival data.
+ */
+class MainUIHandler {
+public:
+  // Callback type for settings button
+  using SettingsButtonCallback = void (*)(void* user_data);
+
+  MainUIHandler();
+  ~MainUIHandler();
+
+  esp_err_t init(
+    lv_obj_t* parent,
+    InteractionHandler* interaction_handler,
+    SettingHandler* setting_handler,
+    NetworkHandler* network_handler
+  );
+  esp_err_t deinit();
+
+  void register_on_settings_button_clicked(SettingsButtonCallback cb, void* user_data);
+  void force_refresh();
+
+private:
+  static void polling_task_(void* param);
+  static void on_settings_button_clicked_static_(lv_event_t* e);
+
+  void on_settings_button_clicked_();
+  void fetch_and_update_arrivals_();
+  std::string format_arrival_time_(const std::string& api_time);
+  std::string get_current_time_string_();
+
+  std::unique_ptr<MainUI> main_ui_;
+  SettingHandler* setting_handler_ = nullptr;
+  NetworkHandler* network_handler_ = nullptr;
+  std::unique_ptr<MTRNextTrainHandler> mtr_handler_;
+
+  // Polling task
+  TaskHandle_t polling_task_handle_ = nullptr;
+  std::atomic<bool> polling_running_{false};
+  SemaphoreHandle_t refresh_mutex_ = nullptr;
+
+  // Callback for settings button
+  SettingsButtonCallback on_settings_callback_ = nullptr;
+  void* settings_callback_user_data_ = nullptr;
+
+  static constexpr uint32_t LVGL_LOCK_TIMEOUT_MS = 4000;
+};
+
+} // namespace travel