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/app.h

@@ -0,0 +1,72 @@
+#pragma once
+
+#include "ui/apps/app.h"
+#include "ui/apps/travel/settings/settings_handler.h"
+#include "ui/apps/travel/ui/main_handler.h"
+#include "ui/apps/travel/ui/settings_handler.h"
+#include "io/nvs_handler.h"
+#include "network/network.h"
+#include <string>
+#include <memory>
+
+// Forward declarations
+namespace travel {
+class MainUIHandler;
+class SettingsUIHandler;
+class SettingHandler;
+}
+
+/**
+ * @brief Travel App - MTR Station Arrival Time Display
+ *
+ * Displays estimated arrival times for configured MTR routes.
+ * Features:
+ * - Support for all MTR lines from assets
+ * - Save up to 5 (station, destination) route pairs
+ * - Poll every 30 seconds (configurable 10-120s)
+ * - Traditional Chinese by default
+ * - E-ink optimized (no animations, static layout)
+ * - Web-based configuration via QR code
+ */
+class TravelApp : public UIApp {
+public:
+  TravelApp();
+  ~TravelApp() override;
+
+  esp_err_t init(lv_obj_t* container, InteractionHandler* interaction_handler) override;
+  esp_err_t deinit(void) override;
+  std::string get_name(void) const override;
+  bool on_back_button_pressed(void) override;
+
+  // Set network handler for API calls
+  void set_network_handler(NetworkHandler* network_handler);
+
+private:
+  // UI handlers
+  std::unique_ptr<travel::MainUIHandler> main_ui_handler_;
+  std::unique_ptr<travel::SettingsUIHandler> settings_ui_handler_;
+
+  // Current page tracking
+  enum class Page {
+    MAIN,
+    SETTINGS
+  };
+  Page current_page_;
+
+  // Settings handler (shared across handlers)
+  std::unique_ptr<travel::SettingHandler> setting_handler_;
+
+  // Network handler (not owned, set externally)
+  NetworkHandler* network_handler_;
+
+  // Interaction handler (not owned)
+  InteractionHandler* interaction_handler_;
+
+  static constexpr const char* NVS_NAMESPACE = "travel_app";
+
+  // Private methods
+  void show_settings_page();
+
+  // UI callback forwarders
+  static void on_settings_button_clicked_static(void* user_data);
+};