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.

Services/SourceRecordingFinder.cs

@@ -0,0 +1,42 @@
+namespace AmiReel.Services;
+
+/// <summary>
+/// Locates sequential WinUAE-style recordings: output.avi, output_1.avi, output_2.avi, ...
+/// Stopping at the first missing name.
+/// </summary>
+public static class SourceRecordingFinder
+{
+    public const string BaseFileName = "output";
+    public const string FileExtension = ".avi";
+    public const int MaximumFiles = 500;
+
+    /// <summary>
+    /// Returns existing sequential recordings in <paramref name="folder"/>, or an empty
+    /// list when the folder is missing or <c>output.avi</c> is not present.
+    /// </summary>
+    public static IReadOnlyList<string> FindSequentialOutputs(string? folder)
+    {
+        if (string.IsNullOrWhiteSpace(folder) || !Directory.Exists(folder))
+            return [];
+
+        string directory = Path.GetFullPath(folder);
+        List<string> found = [];
+
+        string first = Path.Combine(directory, BaseFileName + FileExtension);
+        if (!File.Exists(first))
+            return found;
+
+        found.Add(first);
+
+        for (int index = 1; index < MaximumFiles; index++)
+        {
+            string next = Path.Combine(directory, $"{BaseFileName}_{index}{FileExtension}");
+            if (!File.Exists(next))
+                break;
+
+            found.Add(next);
+        }
+
+        return found;
+    }
+}