fix(daemon): guard start/connect during shutdown

Once a shutdown was under way the app could still launch or restart the
full node and keep running the RPC connection state machine. That could
spawn a daemon process on the way out and surface a spurious connection
failure toast as the window tore down.

- Starting the embedded daemon now bails out immediately while a
  shutdown is in progress, so the node is never (re)launched on exit.
- The connect attempt returns early during shutdown, so there is no
  start/connect try and no stray failure toast on the exit screen.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.cpp

@@ -5769,6 +5769,7 @@ void App::setDaemonStatus(std::string s)
 
 bool App::startEmbeddedDaemon()
 {
+    if (shutting_down_) return false;   // exit in progress: never (re)launch the node
     if (!supportsEmbeddedDaemon()) {
         DEBUG_LOGF("Embedded daemon support unavailable in this build, not starting\n");
         return false;