FFmpeg locator (TASK 4 ship step 2) + licensing: notices, About, LGPL-shared pin

IFfmpegLocator seam that resolves ffmpeg.exe (PATH -> cache -> pinned download),
plus the licensing compliance that makes a paid GA product defensible.

NOTE FOR USERS: this change shows NO difference in the app's behavior except a
new top-bar "About" button (opens THIRD-PARTY-NOTICES.txt). It is scaffolding
for the encoder/streaming work.

- Services/Encoder/: IFfmpegLocator + FfmpegLocator. Pin is BtbN lgpl-shared
  autobuild-2026-08-09-13-03 (NOT gyan.dev/GPL or static: LGPLv2.1 §6 static
  relink material avoided by dynamic linking); extracts ffmpeg.exe + libav*.dll
  via a staging dir so a crash never leaves a partial cache
- THIRD-PARTY-NOTICES.txt: LGPL/BSD/MIT notices + source offer, copied to the
  build output, surfaced by the About button; v1 gate = bundle full license
  texts (TASK 4 requirement 9)
- ai.md "Licensing - do not violate" guardrails (never GPL/nonfree/static/latest-
  tag, never link FFmpeg in, never drop notices); TASKS.md + Services/index.md
  updated
- Tests: FfmpegLocatorTests - hermetic decision-ladder integration test + shared-
  build DLL extraction + edge cases; docs updated (78 tests passing, 0 warnings)

MainWindow.xaml.cs

@@ -1,5 +1,7 @@
 using System.Collections;
 using System.ComponentModel;
+using System.Diagnostics;
+using System.IO;
 using System.Windows;
 using System.Windows.Controls;
 using System.Windows.Input;
@@ -123,6 +125,21 @@ public partial class MainWindow : Window
         }
     }
 
+    private void AboutButton_Click(object sender, RoutedEventArgs e)
+    {
+        // LGPL/BSD/MIT notices ship next to the exe; open in the OS text viewer.
+        var path = Path.Combine(AppContext.BaseDirectory, "THIRD-PARTY-NOTICES.txt");
+        if (!File.Exists(path)) return;
+        try
+        {
+            Process.Start(new ProcessStartInfo(path) { UseShellExecute = true });
+        }
+        catch (Exception ex)
+        {
+            AppLog.Write(ex, "About: failed to open THIRD-PARTY-NOTICES.txt");
+        }
+    }
+
     private void AddSourceButton_Click(object sender, RoutedEventArgs e)
     {
         if (sender is Button { ContextMenu: { } menu } button)