fix: disable action buttons when disconnected, no error spam

- setActionButtonsEnabled() toggles .disabled class on all cleanup/screenshot
  buttons (both chat toolbar and inspector toolbar)
- Called with false in updateConnection when server URL is null
- Called with true when connection established
- runCleanup/runScreenshot silently return when disconnected instead of
  showing 'Not connected' error notifications
- CSS .disabled style: pointer-events:none, opacity:0.3, cursor:not-allowed

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

extension/sidepanel.css

@@ -627,6 +627,12 @@ body::after {
   transform: scale(0.96);
 }
 
+.quick-action-btn.disabled, .inspector-action-btn.disabled {
+  pointer-events: none;
+  opacity: 0.3;
+  cursor: not-allowed;
+}
+
 .quick-action-btn.loading {
   pointer-events: none;
   opacity: 0.5;