feat(text): add static text editor and preview

TextOverlay layers now have a multiline editor, Windows font and size controls, color/emphasis/alignment settings, and a visible draggable preview. Text sources follow OBS's static text-source conventions: https://obsproject.com/kb/text-sources

ytLive.Tests/TextLayerEditorPreviewTests.cs

@@ -0,0 +1,98 @@
+using System;
+using System.IO;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Media;
+using Microsoft.Data.Sqlite;
+using Xunit;
+using ytLive.Controls;
+using ytLive.Models;
+using ytLive.ViewModels;
+
+namespace ytLive.Tests;
+
+[Collection("RealApp")]
+public sealed class TextLayerEditorPreviewTests
+{
+    private readonly RealAppHost _app;
+
+    public TextLayerEditorPreviewTests(RealAppHost app) => _app = app;
+
+    [Fact]
+    public void Editing_One_Text_Layer_Updates_Only_That_Preview()
+    {
+        _app.Run(Run);
+    }
+
+    private static void Run()
+    {
+        var dbPath = Path.Combine(Path.GetTempPath(), $"ytLlive-text-editor-{Guid.NewGuid():N}.db");
+        MainViewModel.LayoutPathOverride = dbPath;
+        MainWindow? window = null;
+        try
+        {
+            window = new MainWindow();
+            var vm = (MainViewModel)window.DataContext;
+            vm.EditingActive = true;
+            vm.StagedScene = vm.Scenes.Single(scene => SceneCatalog.Is(scene.Name, SceneCatalog.Live));
+            vm.AddSourceCommand.Execute(SourceType.TextOverlay);
+            vm.AddSourceCommand.Execute(SourceType.TextOverlay);
+
+            var textLayers = vm.StagedScene!.Elements.OfType<Source>()
+                .Where(source => source.Type == SourceType.TextOverlay).ToArray();
+            Assert.Equal(2, textLayers.Length);
+            var first = textLayers[0];
+            var second = textLayers[1];
+            second.TextContent = "Leave this caption alone";
+            vm.SelectedElement = first;
+
+            window.Show();
+            window.UpdateLayout();
+
+            var leftPanel = (LeftPanel)window.FindName("LeftPanel")!;
+            var editor = (TextBox)leftPanel.FindName("TextLayerEditor")!;
+            Assert.Equal("Text", editor.Text);
+            Assert.True(first.IsDraggable);
+            Assert.Equal(900, first.Width);
+            Assert.Equal(140, first.Height);
+
+            editor.Text = "Updated headline\nSecond line";
+            editor.GetBindingExpression(TextBox.TextProperty)?.UpdateSource();
+            window.UpdateLayout();
+
+            Assert.Equal("Updated headline\nSecond line", first.TextContent);
+            Assert.Equal("Leave this caption alone", second.TextContent);
+
+            var preview = (PreviewPane)window.FindName("PreviewPane")!;
+            var firstPreview = FindTextPreview(preview, first);
+            var secondPreview = FindTextPreview(preview, second);
+            Assert.NotNull(firstPreview);
+            Assert.NotNull(secondPreview);
+            Assert.Equal("Updated headline\nSecond line", firstPreview!.Text);
+            Assert.Equal("Leave this caption alone", secondPreview!.Text);
+            Assert.True(firstPreview.IsVisible);
+            Assert.True(secondPreview.IsVisible);
+        }
+        finally
+        {
+            window?.Close();
+            MainViewModel.LayoutPathOverride = null;
+            SqliteConnection.ClearAllPools();
+            try { File.Delete(dbPath); } catch { /* best-effort cleanup */ }
+        }
+    }
+
+    private static TextBlock? FindTextPreview(DependencyObject parent, Source source)
+    {
+        if (parent is TextBlock text && Equals(text.Tag, "TextLayerPreview")
+            && ReferenceEquals(text.DataContext, source))
+            return text;
+
+        for (var i = 0; i < VisualTreeHelper.GetChildrenCount(parent); i++)
+        {
+            var result = FindTextPreview(VisualTreeHelper.GetChild(parent, i), source);
+            if (result != null) return result;
+        }
+        return null;
+    }
+}