GUI: Discord Rich Presence (#73)

Publish launcher status to Discord over its local IPC named pipe with
no external dependency: framed-JSON handshake and SET_ACTIVITY, a
background worker with reconnection and latest-state dedup, and silent
no-op behavior when Discord is not running.

- "Browsing the library" (game count, session elapsed) while idle;
  "Playing <game>" with title id and elapsed time during emulation.
  Activities always carry timestamps: Discord accepts but does not
  render activities without them
- Presence flips back to browsing immediately on Stop instead of
  waiting for process exit: a game wedged in a GPU driver call can
  outlive termination for a while. Stop also terminates the job
  object so the whole child tree dies even in that state
- Toggle in the Options panel (persisted); client id configurable in
  gui-settings.json; SHARPEMU_LOG_DISCORD=1 traces queue/send/failure

src/SharpEmu.GUI/MainWindow.axaml

@@ -252,10 +252,14 @@ SPDX-License-Identifier: GPL-2.0-or-later
               <TextBlock Classes="fieldLabel" Text="Log to file" />
               <ToggleSwitch x:Name="LogToFileToggle" OnContent="On" OffContent="Off" />
             </StackPanel>
-            <StackPanel>
+            <StackPanel Margin="0,0,24,0">
               <TextBlock Classes="fieldLabel" Text="Title music" />
               <ToggleSwitch x:Name="TitleMusicToggle" OnContent="On" OffContent="Off" IsChecked="True" />
             </StackPanel>
+            <StackPanel>
+              <TextBlock Classes="fieldLabel" Text="Discord presence" />
+              <ToggleSwitch x:Name="DiscordToggle" OnContent="On" OffContent="Off" />
+            </StackPanel>
           </WrapPanel>
         </StackPanel>
       </Border>