fix(tx-history): keep both date bounds when only one filter date changes

lib/screens/transaction_history/cubits/filter/transaction_history_filter_cubit.dart

@@ -37,14 +37,20 @@ class TransactionHistoryFilterCubit extends Cubit<TransactionHistoryFilterState>
   }
 
   void changeFilter({DateTime? startDate, DateTime? endDate}) {
+    // Each date picker reports only the bound it changed, so resolve the
+    // effective window by merging the partial argument with the bound already
+    // in state. Using the merged values for BOTH the state and the filtered
+    // result keeps the other bound from being silently dropped (issue #657 P3).
+    final effectiveStartDate = startDate ?? state.startDate;
+    final effectiveEndDate = endDate ?? state.endDate;
     emit(
       state.copyWith(
-        startDate: startDate,
-        endDate: endDate,
+        startDate: effectiveStartDate,
+        endDate: effectiveEndDate,
         filtered: _applyFilter(
           state.all,
-          startDate: startDate,
-          endDate: endDate,
+          startDate: effectiveStartDate,
+          endDate: effectiveEndDate,
         ),
       ),
     );

test/screens/transaction_history/cubits/transaction_history_filter_cubit_test.dart

@@ -128,6 +128,32 @@ void main() {
       },
     );
 
+    blocTest<TransactionHistoryFilterCubit, TransactionHistoryFilterState>(
+      'changeFilter keeps the previously-set bound when only the other changes '
+      '(issue #657 P3 regression)',
+      build: build,
+      act: (cubit) async {
+        stream.add([
+          _tx(DateTime(2026, 1, 1)),
+          _tx(DateTime(2026, 3, 1)),
+          _tx(DateTime(2026, 5, 1)),
+        ]);
+        await Future<void>.delayed(Duration.zero);
+
+        // Two pickers, two separate partial updates — start first, then end.
+        cubit.changeFilter(startDate: DateTime(2026, 2, 1));
+        cubit.changeFilter(endDate: DateTime(2026, 4, 1));
+      },
+      verify: (cubit) {
+        // Both bounds must survive: the window [Feb 1, Apr 1] leaves only Mar 1.
+        // Before the fix the second call dropped the start bound, leaking Jan 1.
+        expect(cubit.state.startDate, DateTime(2026, 2, 1));
+        expect(cubit.state.endDate, DateTime(2026, 4, 1));
+        expect(cubit.state.filtered, hasLength(1));
+        expect(cubit.state.filtered.single.timestamp, DateTime(2026, 3, 1));
+      },
+    );
+
     test('close cancels the stream subscription (no emit after close)', () async {
       final cubit = build();
       await cubit.close();