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/network/CrossPointWebServer.h

@@ -35,6 +35,7 @@ class CrossPointWebServer {
  private:
   std::unique_ptr<WebServer> server = nullptr;
   bool running = false;
+  bool apMode = false;  // true when running in AP mode, false for STA mode
   uint16_t port = 80;
 
   // File scanning