perf(app): the dashboard works itself out once a burst of changes ends

The home tab stays open below a study session, and every rating changed
what it shows; it now waits until changes pause for 300 ms, so a large
library isn't read anew after each rating.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/src/feature/home/bloc/home_bloc.dart

@@ -57,8 +57,24 @@ class HomeBloc(
   /// out never overwrites a newer one.
   var _refreshes = 0;
 
+  /// Waits for a burst of changes to end, e.g. ratings in a study session
+  /// while this tab stays open below it, so the dashboard is worked out once
+  /// rather than after every one.
+  Timer? _settling;
+
+  /// How long changes must pause before the dashboard is worked out anew.
+  static const settleTime = Duration(milliseconds: 300);
+
   void _onStarted(HomeEventStarted event, Emitter<HomeState> emit) {
-    void changed(Object? _) => add(const HomeEventChanged());
+    void changed(Object? _) {
+      _settling?.cancel();
+      _settling = Timer(settleTime, () {
+        if (!isClosed) {
+          add(const HomeEventChanged());
+        }
+      });
+    }
+
     _subscriptions.addAll([
       _reviews.changes.listen(changed),
       _cards.changes.listen(changed),
@@ -125,6 +141,7 @@ class HomeBloc(
 
   @override
   Future<void> close() async {
+    _settling?.cancel();
     for (final subscription in _subscriptions) {
       await subscription.cancel();
     }