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.

Models/AppSettings.cs

@@ -21,6 +21,12 @@ public sealed record AppSettings
     public required string EndCardHoldSeconds { get; init; }
     public required string ThumbnailInterval { get; init; }
     public bool? MoveSourcesToOriginals { get; init; }
+    public string SourceSearchFolder { get; init; } = "";
+    public string VideoSourceCropMode { get; init; } = "Auto";
+    public string VideoCropLeft { get; init; } = "0";
+    public string VideoCropRight { get; init; } = "0";
+    public string VideoCropTop { get; init; } = "0";
+    public string VideoCropBottom { get; init; } = "0";
 
     // Shorts tab defaults (sticky across launches, same as the fields above).
     public string ShortsStyle { get; init; } = "Brand";
@@ -57,6 +63,12 @@ public sealed record AppSettings
         EndCardHoldSeconds = "4",
         ThumbnailInterval = "10",
         MoveSourcesToOriginals = true,
+        SourceSearchFolder = "",
+        VideoSourceCropMode = "Auto",
+        VideoCropLeft = "0",
+        VideoCropRight = "0",
+        VideoCropTop = "0",
+        VideoCropBottom = "0",
     };
 
     public AppSettings Normalize(string fallbackOutputFolder) => this with
@@ -73,6 +85,12 @@ public sealed record AppSettings
         EndCardHoldSeconds = string.IsNullOrWhiteSpace(EndCardHoldSeconds) ? "4" : EndCardHoldSeconds,
         ThumbnailInterval = string.IsNullOrWhiteSpace(ThumbnailInterval) ? "10" : ThumbnailInterval,
         MoveSourcesToOriginals = MoveSourcesToOriginals ?? true,
+        SourceSearchFolder = SourceSearchFolder ?? "",
+        VideoSourceCropMode = string.IsNullOrWhiteSpace(VideoSourceCropMode) ? "Auto" : VideoSourceCropMode,
+        VideoCropLeft = string.IsNullOrWhiteSpace(VideoCropLeft) ? "0" : VideoCropLeft,
+        VideoCropRight = string.IsNullOrWhiteSpace(VideoCropRight) ? "0" : VideoCropRight,
+        VideoCropTop = string.IsNullOrWhiteSpace(VideoCropTop) ? "0" : VideoCropTop,
+        VideoCropBottom = string.IsNullOrWhiteSpace(VideoCropBottom) ? "0" : VideoCropBottom,
         ShortsStyle = string.IsNullOrWhiteSpace(ShortsStyle) ? "Brand" : ShortsStyle,
         ShortsHook = ShortsHook ?? "THIS RAN ON AN AMIGA",
         ShortsWebsite = ShortsWebsite ?? "AMIGADB.NET",