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/FfmpegOutputFilterTests.cs

@@ -16,6 +16,8 @@ public class FfmpegOutputFilterTests
     [DataRow("Stream #0:0: Video: mjpeg")]
     [DataRow("Duration: 00:01:00.00, start: 0.000000, bitrate: 128 kb/s")]
     [DataRow("video:1024kB audio:128kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.5%")]
+    [DataRow("Last message repeated 1 times")]
+    [DataRow("Last message repeated 24 times")]
     public void IsBoilerplateLine_WithKnownFfmpegBanner_ReturnsTrue(string line)
     {
         // Act