Initial commit: FieldLogger MAUI app with Maglink BLE support

Added Maglink RTK GNSS receiver integration with correct BLE UUIDs and device name filtering (ML-* prefix).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

FieldLogger/Models/DeviceKind.cs

@@ -0,0 +1,11 @@
+namespace FieldLogger.Models;
+
+/// <summary>The two kinds of BLE devices the app pairs with.</summary>
+public enum DeviceKind
+{
+    /// <summary>Underground Magnetics utility locating receiver (UMRX_* / DT100_*).</summary>
+    Locator,
+
+    /// <summary>Maglink RTK GNSS receiver (H11).</summary>
+    RtkGps,
+}