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/Services/Sync/IMqttSyncService.cs

@@ -0,0 +1,27 @@
+using FieldLogger.Models;
+
+namespace FieldLogger.Services.Sync;
+
+/// <summary>
+/// Placeholder for the future MQTT synchronization layer:
+/// - subscribe to receive jobs configured on the server
+/// - publish logged points as they are captured
+/// - reconcile the Synced flags on <see cref="Job"/> and <see cref="LoggedPoint"/>
+/// Planned implementation: MQTTnet client against the configured broker.
+/// </summary>
+public interface IMqttSyncService
+{
+    bool IsConnected { get; }
+    Task ConnectAsync(CancellationToken cancellationToken = default);
+    Task DisconnectAsync();
+    Task PublishPointAsync(LoggedPoint point, CancellationToken cancellationToken = default);
+}
+
+/// <summary>No-op stand-in until the MQTT backend exists.</summary>
+public sealed class NullMqttSyncService : IMqttSyncService
+{
+    public bool IsConnected => false;
+    public Task ConnectAsync(CancellationToken cancellationToken = default) => Task.CompletedTask;
+    public Task DisconnectAsync() => Task.CompletedTask;
+    public Task PublishPointAsync(LoggedPoint point, CancellationToken cancellationToken = default) => Task.CompletedTask;
+}