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/SourceCropFilters.cs

@@ -0,0 +1,113 @@
+using System.Globalization;
+using System.Text.RegularExpressions;
+using AmiReel.Models;
+
+namespace AmiReel.Services;
+
+/// <summary>Shared FFmpeg crop filters used by both Video and Shorts pipelines.</summary>
+internal static class SourceCropFilters
+{
+    internal const string CenterFourByThreeFilter =
+        "crop='min(iw,ih*4/3)':'min(ih,iw*3/4)':'(iw-ow)/2':'(ih-oh)/2'";
+
+    private static readonly Regex CropSpecRegex = new(
+        @"crop=(\d+):(\d+):(\d+):(\d+)",
+        RegexOptions.CultureInvariant);
+
+    internal static string BuildManualCropFilter(int left, int right, int top, int bottom) =>
+        $"crop=iw-{left}-{right}:ih-{top}-{bottom}:{left}:{top}";
+
+    internal static bool HasManualCrop(int left, int right, int top, int bottom) =>
+        left > 0 || right > 0 || top > 0 || bottom > 0;
+
+    internal static string Resolve(
+        ShortSourceCropMode mode,
+        int left,
+        int right,
+        int top,
+        int bottom,
+        string? autoDetected)
+    {
+        if (HasManualCrop(left, right, top, bottom))
+            return BuildManualCropFilter(left, right, top, bottom);
+
+        return mode switch
+        {
+            ShortSourceCropMode.None => "",
+            ShortSourceCropMode.Auto when !string.IsNullOrWhiteSpace(autoDetected) => autoDetected,
+            ShortSourceCropMode.Auto => CenterFourByThreeFilter,
+            _ => CenterFourByThreeFilter,
+        };
+    }
+
+    internal static string Prefix(string cropFilter, string nextFilter) =>
+        string.IsNullOrWhiteSpace(cropFilter) ? nextFilter : $"{cropFilter},{nextFilter}";
+
+    internal static List<string> BuildHorizontalCropDetectArguments(string inputFile, string start, double durationSeconds) =>
+    [
+        "-hide_banner",
+        "-ss",
+        string.IsNullOrWhiteSpace(start) ? "00:00:00" : start,
+        "-t",
+        durationSeconds.ToString("0.###", CultureInfo.InvariantCulture),
+        "-i",
+        inputFile,
+        "-an",
+        "-vf",
+        "crop=iw:trunc(ih*0.70/2)*2:0:trunc(ih*0.15/2)*2,cropdetect=limit=28:round=2:reset=0",
+        "-f",
+        "null",
+        "-",
+    ];
+
+    internal static string? SelectMostCommonCropSpec(string ffmpegOutput)
+    {
+        Dictionary<string, int> counts = new(StringComparer.Ordinal);
+        foreach (Match match in CropSpecRegex.Matches(ffmpegOutput))
+        {
+            string spec = match.Value;
+            counts[spec] = counts.GetValueOrDefault(spec) + 1;
+        }
+
+        return counts.Count == 0
+            ? null
+            : counts.OrderByDescending(pair => pair.Value).ThenBy(pair => pair.Key, StringComparer.Ordinal).First().Key;
+    }
+
+    internal static bool TryParseCropSpec(string spec, out int width, out int height, out int x, out int y)
+    {
+        Match match = CropSpecRegex.Match(spec);
+        if (!match.Success)
+        {
+            width = height = x = y = 0;
+            return false;
+        }
+
+        width = int.Parse(match.Groups[1].Value, CultureInfo.InvariantCulture);
+        height = int.Parse(match.Groups[2].Value, CultureInfo.InvariantCulture);
+        x = int.Parse(match.Groups[3].Value, CultureInfo.InvariantCulture);
+        y = int.Parse(match.Groups[4].Value, CultureInfo.InvariantCulture);
+        return true;
+    }
+
+    internal static bool TryBuildAutoHorizontalCrop(int detectedWidth, int detectedX, out int width, out int x)
+    {
+        int inset = (int)(detectedWidth * 0.005 / 2) * 2;
+        width = ((detectedWidth - (2 * inset)) / 2) * 2;
+        x = ((detectedX + inset) / 2) * 2;
+        if (width < 16)
+            return false;
+        return true;
+    }
+
+    internal static string? TryBuildAutoHorizontalFilter(string ffmpegOutput)
+    {
+        string? spec = SelectMostCommonCropSpec(ffmpegOutput);
+        if (spec is null || !TryParseCropSpec(spec, out int width, out _, out int x, out _))
+            return null;
+        if (!TryBuildAutoHorizontalCrop(width, x, out int cropWidth, out int cropX))
+            return null;
+
+        return $"crop={cropWidth}:ih:{cropX}:0";
+    }
+}