initial

State.cs

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+using System;
+using System.IO;
+using System.Text.Json;
+
+namespace Bedtimer
+{
+    public static class State
+    {
+        private sealed class Model
+        {
+            public string? LastLockCycleId { get; set; }
+            public DateTime LastLockUtc { get; set; }
+        }
+
+        private static string PathJson => System.IO.Path.Combine(Config.Dir, "state.json");
+        private static readonly object _gate = new();
+
+        private static Model Load()
+        {
+            lock (_gate)
+            {
+                Directory.CreateDirectory(Path.GetDirectoryName(PathJson)!);
+                if (!File.Exists(PathJson)) return new Model();
+                try { return JsonSerializer.Deserialize<Model>(File.ReadAllText(PathJson)) ?? new Model(); }
+                catch { return new Model(); }
+            }
+        }
+
+        private static void Save(Model m)
+        {
+            lock (_gate)
+            {
+                Directory.CreateDirectory(Path.GetDirectoryName(PathJson)!);
+                File.WriteAllText(PathJson, JsonSerializer.Serialize(m, new JsonSerializerOptions { WriteIndented = true }));
+            }
+        }
+
+        public static bool IsLockedForCycle(string cycleId)
+        {
+            var m = Load();
+            return string.Equals(m.LastLockCycleId, cycleId, StringComparison.Ordinal);
+        }
+
+        public static void MarkLocked(string cycleId)
+        {
+            Save(new Model { LastLockCycleId = cycleId, LastLockUtc = DateTime.UtcNow });
+        }
+    }
+}