Add video preview, source crop, find-videos, and render toasts

Bring the Video tab closer to Shorts with 4K preview stills and crop
modes (none, auto, center, plus the visual editor). Settings store a
default source folder so Find videos can collect output.avi sequences.
Cap the FFmpeg log with auto-scroll, hide repeat spam, and show a
Windows notification when a video or short finishes or fails.

AmiReel.Tests/Services/SourceRecordingFinderTests.cs

@@ -0,0 +1,99 @@
+using AmiReel.Services;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AmiReel.Tests.Services;
+
+[TestClass]
+public class SourceRecordingFinderTests
+{
+    private readonly List<string> _tempDirectories = [];
+
+    [TestCleanup]
+    public void Cleanup()
+    {
+        foreach (string directory in _tempDirectories)
+        {
+            try { Directory.Delete(directory, true); } catch { }
+        }
+    }
+
+    private string CreateTempDirectory()
+    {
+        string directory = Path.Combine(Path.GetTempPath(), "AmiReelTests", Guid.NewGuid().ToString("N"));
+        Directory.CreateDirectory(directory);
+        _tempDirectories.Add(directory);
+        return directory;
+    }
+
+    [TestMethod]
+    public void FindSequentialOutputs_WithMissingFolder_ReturnsEmpty()
+    {
+        // Act
+        IReadOnlyList<string> result = SourceRecordingFinder.FindSequentialOutputs(
+            Path.Combine(Path.GetTempPath(), "AmiReelTests", "missing-" + Guid.NewGuid().ToString("N")));
+
+        // Assert
+        Assert.AreEqual(0, result.Count);
+    }
+
+    [TestMethod]
+    public void FindSequentialOutputs_WithBlankFolder_ReturnsEmpty()
+    {
+        // Act
+        IReadOnlyList<string> result = SourceRecordingFinder.FindSequentialOutputs("   ");
+
+        // Assert
+        Assert.AreEqual(0, result.Count);
+    }
+
+    [TestMethod]
+    public void FindSequentialOutputs_WithOnlyOutputAvi_ReturnsSingleFile()
+    {
+        // Arrange
+        string directory = CreateTempDirectory();
+        string first = Path.Combine(directory, "output.avi");
+        File.WriteAllText(first, "one");
+
+        // Act
+        IReadOnlyList<string> result = SourceRecordingFinder.FindSequentialOutputs(directory);
+
+        // Assert
+        Assert.AreEqual(1, result.Count);
+        Assert.AreEqual(first, result[0]);
+    }
+
+    [TestMethod]
+    public void FindSequentialOutputs_WithNumberedSequence_ReturnsFilesUntilFirstGap()
+    {
+        // Arrange
+        string directory = CreateTempDirectory();
+        File.WriteAllText(Path.Combine(directory, "output.avi"), "0");
+        File.WriteAllText(Path.Combine(directory, "output_1.avi"), "1");
+        File.WriteAllText(Path.Combine(directory, "output_2.avi"), "2");
+        File.WriteAllText(Path.Combine(directory, "output_4.avi"), "skipped");
+        File.WriteAllText(Path.Combine(directory, "output.mp4"), "wrong-ext");
+
+        // Act
+        IReadOnlyList<string> result = SourceRecordingFinder.FindSequentialOutputs(directory);
+
+        // Assert
+        Assert.AreEqual(3, result.Count);
+        Assert.AreEqual("output.avi", Path.GetFileName(result[0]));
+        Assert.AreEqual("output_1.avi", Path.GetFileName(result[1]));
+        Assert.AreEqual("output_2.avi", Path.GetFileName(result[2]));
+    }
+
+    [TestMethod]
+    public void FindSequentialOutputs_WithoutBaseOutputAvi_ReturnsEmptyEvenIfNumberedExists()
+    {
+        // Arrange
+        string directory = CreateTempDirectory();
+        File.WriteAllText(Path.Combine(directory, "output_1.avi"), "1");
+
+        // Act
+        IReadOnlyList<string> result = SourceRecordingFinder.FindSequentialOutputs(directory);
+
+        // Assert
+        Assert.AreEqual(0, result.Count);
+    }
+}