fix: prevent timer memory leak in spell check

Cancel all pending debounce timers in dispose() to prevent
memory leaks and potential crashes when the widget is disposed
during active spell checking.

Author: LucasXu0

lib/src/editor/block_component/rich_text/appflowy_rich_text.dart

@@ -145,6 +145,16 @@ class _AppFlowyRichTextState extends State<AppFlowyRichText>
     confirmContextEnabled();
   }
 
+  @override
+  void dispose() {
+    // Cancel all pending spell check timers to prevent memory leaks
+    for (final timer in _debounceTimers.values) {
+      timer?.cancel();
+    }
+    _debounceTimers.clear();
+    super.dispose();
+  }
+
   @override
   Widget build(BuildContext context) {
     Widget child = Stack(