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/RenderNotifierTests.cs

@@ -0,0 +1,67 @@
+using AmiReel.Services;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace AmiReel.Tests.Services;
+
+[TestClass]
+public class RenderNotifierTests
+{
+    [TestMethod]
+    public void DescribeCompleted_ForVideo_UsesRenderCompleteCopy()
+    {
+        // Act
+        (string title, string body) = RenderNotifier.DescribeCompleted(shorts: false);
+
+        // Assert
+        Assert.AreEqual("Render complete", title);
+        StringAssert.Contains(body, "successfully");
+    }
+
+    [TestMethod]
+    public void DescribeCompleted_ForShorts_UsesShortCompleteCopy()
+    {
+        // Act
+        (string title, string body) = RenderNotifier.DescribeCompleted(shorts: true);
+
+        // Assert
+        Assert.AreEqual("Short complete", title);
+        StringAssert.Contains(body, "Short");
+    }
+
+    [TestMethod]
+    public void DescribeFailed_WithBlankSummary_UsesFallbackBody()
+    {
+        // Act
+        (string title, string body) = RenderNotifier.DescribeFailed("   ");
+
+        // Assert
+        Assert.AreEqual("Render failed", title);
+        Assert.AreEqual("The render failed.", body);
+    }
+
+    [TestMethod]
+    public void DescribeFailed_WithLongSummary_TruncatesBody()
+    {
+        // Arrange
+        string summary = new('x', 250);
+
+        // Act
+        (string title, string body) = RenderNotifier.DescribeFailed(summary);
+
+        // Assert
+        Assert.AreEqual("Render failed", title);
+        Assert.AreEqual(180, body.Length);
+        StringAssert.EndsWith(body, "...");
+    }
+
+    [TestMethod]
+    public void DescribeCancelled_ReturnsCancelledCopy()
+    {
+        // Act
+        (string title, string body) = RenderNotifier.DescribeCancelled();
+
+        // Assert
+        Assert.AreEqual("Render cancelled", title);
+        StringAssert.Contains(body, "cancelled");
+    }
+}