Add host activity and DB browser, and keep dialogs, drag-drop, and idle maintenance responsive.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/Explorer.Application/HostActivityLog.cs

@@ -0,0 +1,87 @@
+using Explorer.Contracts;
+
+namespace Explorer.Application;
+
+public interface IHostActivitySink
+{
+    void Record(string category, string message);
+    IReadOnlyList<HostActivityEvent> TakeRecent(int max = 120);
+}
+
+public sealed class NullHostActivitySink : IHostActivitySink
+{
+    public static NullHostActivitySink Instance { get; } = new();
+
+    public void Record(string category, string message)
+    {
+    }
+
+    public IReadOnlyList<HostActivityEvent> TakeRecent(int max = 120) => [];
+}
+
+/// <summary>Bounded ring of host activity lines for the live monitor. Cheap to write; snapshot is a copy.</summary>
+public sealed class HostActivityLog : IHostActivitySink
+{
+    private readonly object _gate = new();
+    private readonly HostActivityEvent[] _ring;
+    private int _next;
+    private int _count;
+    private string? _lastKey;
+    private DateTimeOffset _lastUtc;
+
+    public HostActivityLog(int capacity = 48)
+    {
+        _ring = new HostActivityEvent[Math.Clamp(capacity, 32, 200)];
+    }
+
+    public void Record(string category, string message)
+    {
+        if (string.IsNullOrWhiteSpace(message))
+        {
+            return;
+        }
+
+        var cat = string.IsNullOrWhiteSpace(category) ? "Host" : category.Trim();
+        var msg = message.Trim();
+        var key = cat + "\u001f" + msg;
+        var now = DateTimeOffset.UtcNow;
+        lock (_gate)
+        {
+            if (key == _lastKey && (now - _lastUtc) < TimeSpan.FromMilliseconds(750))
+            {
+                return;
+            }
+
+            _lastKey = key;
+            _lastUtc = now;
+            _ring[_next] = new HostActivityEvent { Utc = now, Category = cat, Message = msg };
+            _next = (_next + 1) % _ring.Length;
+            if (_count < _ring.Length)
+            {
+                _count++;
+            }
+        }
+    }
+
+    public IReadOnlyList<HostActivityEvent> TakeRecent(int max = 120)
+    {
+        max = Math.Clamp(max, 1, _ring.Length);
+        lock (_gate)
+        {
+            var take = Math.Min(max, _count);
+            if (take == 0)
+            {
+                return [];
+            }
+
+            var result = new HostActivityEvent[take];
+            var start = (_next - take + _ring.Length) % _ring.Length;
+            for (var i = 0; i < take; i++)
+            {
+                result[i] = _ring[(start + i) % _ring.Length];
+            }
+
+            return result;
+        }
+    }
+}