Add AP mode option for file transfers (#98)

## Summary

* **What is the goal of this PR?** Adds WiFi Access Point (AP) mode
support for File Transfer, allowing the device to create its own WiFi
network that users can connect to directly - useful when no existing
WiFi network is available. And in my experience is faster when the
device is right next to your laptop (but maybe further from your wifi)

* **What changes are included?**
- New `NetworkModeSelectionActivity` - an interstitial screen asking
users to choose between:
- "Join a Network" - connects to an existing WiFi network (existing
behavior)
- "Create Hotspot" - creates a WiFi access point named
"CrossPoint-Reader"
  - Modified `CrossPointWebServerActivity` to:
    - Launch the network mode selection screen before proceeding
- Support starting an Access Point with mDNS (`crosspoint.local`) and
DNS server for captive portal behavior
    - Display appropriate connection info for both modes
- Modified `CrossPointWebServer` to support starting when WiFi is in AP
mode (not just STA connected mode)

## Additional Context

* **AP Mode Details**: The device creates an open WiFi network named
"CrossPoint-Reader". Once connected, users can access the file transfer
page at `http://crosspoint.local/` or `http://192.168.4.1/`
* **DNS Captive Portal**: A DNS server redirects all domain requests to
the device's IP, enabling captive portal behavior on some devices
* **mDNS**: Hostname resolution via `crosspoint.local` is enabled for
both AP and STA modes
* **No breaking changes**: The "Join a Network" option preserves the
existing WiFi connection flow
* **Memory impact**: Minimal - the AP mode uses roughly the same
resources as STA mode

src/activities/network/CrossPointWebServerActivity.h

@@ -7,12 +7,15 @@
 #include <memory>
 #include <string>
 
+#include "NetworkModeSelectionActivity.h"
 #include "activities/ActivityWithSubactivity.h"
 #include "network/CrossPointWebServer.h"
 
 // Web server activity states
 enum class WebServerActivityState {
-  WIFI_SELECTION,  // WiFi selection subactivity is active
+  MODE_SELECTION,  // Choosing between Join Network and Create Hotspot
+  WIFI_SELECTION,  // WiFi selection subactivity is active (for Join Network mode)
+  AP_STARTING,     // Starting Access Point mode
   SERVER_RUNNING,  // Web server is running and handling requests
   SHUTTING_DOWN    // Shutting down server and WiFi
 };
@@ -20,8 +23,10 @@ enum class WebServerActivityState {
 /**
  * CrossPointWebServerActivity is the entry point for file transfer functionality.
  * It:
- * - Immediately turns on WiFi and launches WifiSelectionActivity on enter
- * - When WifiSelectionActivity completes successfully, starts the CrossPointWebServer
+ * - First presents a choice between "Join a Network" (STA) and "Create Hotspot" (AP)
+ * - For STA mode: Launches WifiSelectionActivity to connect to an existing network
+ * - For AP mode: Creates an Access Point that clients can connect to
+ * - Starts the CrossPointWebServer when connected
  * - Handles client requests in its loop() function
  * - Cleans up the server and shuts down WiFi on exit
  */
@@ -29,15 +34,19 @@ class CrossPointWebServerActivity final : public ActivityWithSubactivity {
   TaskHandle_t displayTaskHandle = nullptr;
   SemaphoreHandle_t renderingMutex = nullptr;
   bool updateRequired = false;
-  WebServerActivityState state = WebServerActivityState::WIFI_SELECTION;
+  WebServerActivityState state = WebServerActivityState::MODE_SELECTION;
   const std::function<void()> onGoBack;
 
+  // Network mode
+  NetworkMode networkMode = NetworkMode::JOIN_NETWORK;
+  bool isApMode = false;
+
   // Web server - owned by this activity
   std::unique_ptr<CrossPointWebServer> webServer;
 
   // Server status
   std::string connectedIP;
-  std::string connectedSSID;
+  std::string connectedSSID;  // For STA mode: network name, For AP mode: AP name
 
   // Performance monitoring
   unsigned long lastHandleClientTime = 0;
@@ -47,7 +56,9 @@ class CrossPointWebServerActivity final : public ActivityWithSubactivity {
   void render() const;
   void renderServerRunning() const;
 
+  void onNetworkModeSelected(NetworkMode mode);
   void onWifiSelectionComplete(bool connected);
+  void startAccessPoint();
   void startWebServer();
   void stopWebServer();