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/NetworkModeSelectionActivity.h

@@ -0,0 +1,41 @@
+#pragma once
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include <functional>
+
+#include "../Activity.h"
+
+// Enum for network mode selection
+enum class NetworkMode { JOIN_NETWORK, CREATE_HOTSPOT };
+
+/**
+ * NetworkModeSelectionActivity presents the user with a choice:
+ * - "Join a Network" - Connect to an existing WiFi network (STA mode)
+ * - "Create Hotspot" - Create an Access Point that others can connect to (AP mode)
+ *
+ * The onModeSelected callback is called with the user's choice.
+ * The onCancel callback is called if the user presses back.
+ */
+class NetworkModeSelectionActivity final : public Activity {
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  int selectedIndex = 0;
+  bool updateRequired = false;
+  const std::function<void(NetworkMode)> onModeSelected;
+  const std::function<void()> onCancel;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void render() const;
+
+ public:
+  explicit NetworkModeSelectionActivity(GfxRenderer& renderer, InputManager& inputManager,
+                                        const std::function<void(NetworkMode)>& onModeSelected,
+                                        const std::function<void()>& onCancel)
+      : Activity("NetworkModeSelection", renderer, inputManager), onModeSelected(onModeSelected), onCancel(onCancel) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};