Add graceful shutdown button and improve shutdown status display

- Add refresh/shutdown button next to status indicator that gracefully
  stops the process without cleaning my-storage
- Create /api/shutdown endpoint that triggers graceful shutdown via SIGTERM
- Update status indicator to show "Gracefully Cleaning...." when system
  is shutting down (instead of "Searching for peers...")
- Include isShuttingDown flag in /api/status endpoint response
- Use built-in ConfirmationModal for shutdown confirmation with warning styling

The shutdown button appears in the top-right corner next to the status
indicator and uses the existing confirmation modal system for user
confirmation before initiating shutdown.

includes/admin/admin-frontend/index.html

@@ -32,7 +32,12 @@
   <div class="container mx-auto max-w-7xl">
     <div class="flex justify-between items-center mb-8 flex-wrap gap-4">
       <h1 class="text-4xl font-extrabold" style="background: linear-gradient(135deg, var(--primary) 0%, var(--secondary) 100%); -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text;">P2NS Admin Panel</h1>
-      <div id="status-indicator" class="px-4 py-2 rounded-lg"></div>
+      <div class="flex items-center gap-2">
+        <div id="status-indicator" class="px-4 py-2 rounded-lg"></div>
+        <button id="refresh-button" onclick="handleRefresh()" class="px-3 py-2 bg-primary hover:bg-primary-hover text-white rounded-lg transition-colors" title="Gracefully stop the process">
+          <i class="fas fa-redo"></i>
+        </button>
+      </div>
     </div>
   
     <nav class="flex justify-center mb-8 space-x-4 flex-wrap">