feat(web): add shutdown button to Quick Actions (#234)

Add a "Shutdown System" button to the Overview page that gracefully
powers off the Raspberry Pi. Uses sudo poweroff, consistent with the
existing reboot_system action, letting sudo's secure_path handle
binary resolution.

Co-authored-by: Chuck <chuck@example.com>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

web_interface/blueprints/api_v3.py

@@ -1339,6 +1339,9 @@ def execute_system_action():
         elif action == 'reboot_system':
             result = subprocess.run(['sudo', 'reboot'],
                                  capture_output=True, text=True)
+        elif action == 'shutdown_system':
+            result = subprocess.run(['sudo', 'poweroff'],
+                                 capture_output=True, text=True)
         elif action == 'git_pull':
             # Use PROJECT_ROOT instead of hardcoded path
             project_dir = str(PROJECT_ROOT)