Add Shorts wizard and v4 video-style pipeline

Turn the Shorts tab into a step-by-step wizard and port
amigadb-short-fx-v4.sh: Video background style, auto/manual crop,
source fade-in, and blur/opacity/margin settings.

Models/ShortSettings.cs

@@ -1,6 +1,6 @@
 namespace AmiReel.Models;
 
-/// <summary>Visual style applied to a YouTube Short. Mirrors the --style values of amigadb-short.sh.</summary>
+/// <summary>Visual style applied to a YouTube Short. Mirrors the --style values of amigadb-short-fx-v4.sh.</summary>
 public enum ShortStyle
 {
     Brand,
@@ -22,9 +22,17 @@ public enum ShortStyle
     Monitor,
     Split,
     Spectrum,
+    Video,
 }
 
-/// <summary>One YouTube Shorts render job's parameters (port of amigadb-short.sh's arguments).</summary>
+/// <summary>How the video style crops the Amiga source before overlaying it.</summary>
+public enum ShortSourceCropMode
+{
+    Auto,
+    Center,
+}
+
+/// <summary>One YouTube Shorts render job's parameters (port of amigadb-short-fx-v4.sh).</summary>
 public sealed record ShortSettings
 {
     public required string InputFile { get; init; }
@@ -39,6 +47,16 @@ public sealed record ShortSettings
     public required double DurationSeconds { get; init; }
     public required ShortStyle Style { get; init; }
     public string BackgroundImage { get; init; } = "";
+    public string BackgroundVideo { get; init; } = "";
+    public double VideoBlur { get; init; }
+    public double VideoOpacity { get; init; } = 1.0;
+    public ShortSourceCropMode SourceCropMode { get; init; } = ShortSourceCropMode.Auto;
+    public int CropLeft { get; init; }
+    public int CropRight { get; init; }
+    public int CropTop { get; init; }
+    public int CropBottom { get; init; }
+    public int SourceMarginX { get; init; } = 40;
+    public double SourceFadeInSeconds { get; init; } = 1.5;
     public required string FontFile { get; init; }
     public required string FfmpegPath { get; init; }
     public required string FfprobePath { get; init; }