TASK 18: manual rename modal for finished recordings

RenameRecordingDialog (themed, mirrors GoLiveWindow chrome): pre-fills the auto stem,
Enter/OK saves, Esc/Cancel keeps the auto name, strips stray .mp4, rejects invalid
filename chars. FinalizeRecordingAsync shows it before UniquePath rename. Naming logic
still covered by RecordingFileTests; UI shell is pure. verify.sh gate: 0 warnings,
244/246 (2 known).

RenameRecordingDialog.xaml.cs

@@ -0,0 +1,63 @@
+using System.IO;
+using System.Windows;
+using System.Windows.Input;
+
+namespace ytLive;
+
+/// <summary>Modal that lets the creator rename an auto-named recording before it is
+/// finalized. Returns the chosen file stem (no extension) via <see cref="ResultFileName"/>,
+/// or <c>null</c> on Cancel / keep-auto-name.</summary>
+public partial class RenameRecordingDialog : Window
+{
+    private readonly string _autoStem;
+
+    /// <summary>Stem (no extension) the creator chose, or <c>null</c> to keep auto-name.</summary>
+    public string? ResultFileName { get; private set; }
+
+    public RenameRecordingDialog(string autoStem)
+    {
+        InitializeComponent();
+        _autoStem = autoStem;
+        NameBox.Text = autoStem;
+        Loaded += (_, _) => { NameBox.Focus(); NameBox.SelectAll(); };
+        NameBox.KeyDown += OnKeyDown;
+    }
+
+    private void OnKeyDown(object sender, KeyEventArgs e)
+    {
+        if (e.Key == Key.Enter) { Save(); e.Handled = true; }
+        else if (e.Key == Key.Escape) { DialogResult = false; e.Handled = true; }
+    }
+
+    private void Cancel_Click(object sender, RoutedEventArgs e) => DialogResult = false;
+
+    private void Save_Click(object sender, RoutedEventArgs e) => Save();
+
+    private void Save()
+    {
+        var name = NameBox.Text?.Trim();
+        if (string.IsNullOrEmpty(name))
+        {
+            // Empty = keep the auto name.
+            ResultFileName = null;
+            DialogResult = true;
+            return;
+        }
+
+        // Strip any accidental ".mp4" so we return a clean stem.
+        if (name.EndsWith(".mp4", StringComparison.OrdinalIgnoreCase))
+            name = Path.GetFileNameWithoutExtension(name);
+
+        // Reject characters that are illegal in Windows file names.
+        if (name.IndexOfAny(Path.GetInvalidFileNameChars()) >= 0)
+        {
+            MessageBox.Show(this, "That name contains characters not allowed in file names.",
+                "Invalid name", MessageBoxButton.OK, MessageBoxImage.Warning);
+            NameBox.Focus();
+            return;
+        }
+
+        ResultFileName = name;
+        DialogResult = true;
+    }
+}