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

@@ -105,6 +105,8 @@
 
                 <!-- Single three-state action button -->
                 <StackPanel Grid.Column="2" Orientation="Horizontal" VerticalAlignment="Center">
+                    <Button Content="About" Style="{StaticResource YtButtonSecondary}"
+                            Click="AboutButton_Click" Margin="0,0,12,0" VerticalAlignment="Center"/>
                     <Border Width="26" Height="26" CornerRadius="13" Background="#16213e" ClipToBounds="True"
                             Margin="0,0,8,0" VerticalAlignment="Center"
                             ToolTip="{Binding AccountDisplayName}"