feat: record split mode/percentage changes in history diff

src/main/java/de/zendric/app/xpensely_server/services/HistoryService.java

@@ -69,6 +69,10 @@ public class HistoryService {
         addIfChanged(changes, "personalUseAmount", before.getPersonalUseAmount(), after.getPersonalUseAmount());
         addIfChanged(changes, "otherPersonAmount", before.getOtherPersonAmount(), after.getOtherPersonAmount());
         addIfChanged(changes, "category", before.getCategory(), after.getCategory());
+        addIfChanged(changes, "splitMode",
+                before.getSplitMode() == null ? null : before.getSplitMode().name(),
+                after.getSplitMode() == null ? null : after.getSplitMode().name());
+        addIfChanged(changes, "splitPercentage", before.getSplitPercentage(), after.getSplitPercentage());
         addIfChanged(changes, "date", before.getDate(), after.getDate());
         addIfChanged(changes, "owner",
                 before.getOwner() == null ? null : before.getOwner().getUsername(),

src/test/java/de/zendric/app/xpensely_Server/services/HistoryServiceTest.java

@@ -24,6 +24,7 @@ import de.zendric.app.xpensely_server.model.ExpenseHistoryEntry;
 import de.zendric.app.xpensely_server.model.ExpenseList;
 import de.zendric.app.xpensely_server.model.FieldChange;
 import de.zendric.app.xpensely_server.model.HistoryEntryType;
+import de.zendric.app.xpensely_server.model.SplitMode;
 import de.zendric.app.xpensely_server.repo.ExpenseHistoryRepository;
 import de.zendric.app.xpensely_server.services.HistoryService;
 
@@ -86,6 +87,22 @@ class HistoryServiceTest {
         assertThat(changes).containsExactly(new FieldChange("owner", "alice", "ben"));
     }
 
+    @Test
+    void diff_reportsSplitModeAndPercentageChanges() {
+        Expense before = new Expense();
+        before.setSplitMode(SplitMode.DETAILED);
+        before.setSplitPercentage(null);
+
+        Expense after = new Expense();
+        after.setSplitMode(SplitMode.PERCENTAGE);
+        after.setSplitPercentage(70);
+
+        List<FieldChange> changes = service.diff(before, after);
+
+        assertThat(changes).contains(new FieldChange("splitMode", "DETAILED", "PERCENTAGE"));
+        assertThat(changes).contains(new FieldChange("splitPercentage", null, "70"));
+    }
+
     @Test
     void diff_nullToValue_isReported() {
         Expense before = expense("Taxi", 9.0, null, LocalDate.of(2026, 7, 1), user(1L, "alice"));