fix: allow editing quick-add stub expenses (amount 0, no category)

Quick Add deliberately creates stub expenses with amount 0 and no
category (ExpenseInput allows both), but ExpenseChangeRequest required
amount >= 0.01 and a non-blank category, so every edit of a stub that
did not simultaneously fill in both fields was rejected with 400.
Update validation now accepts the same states create can produce, and
a blank category is normalized to null.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/java/de/zendric/app/xpensely_server/model/ExpenseChangeRequest.java

@@ -24,8 +24,10 @@ public class ExpenseChangeRequest {
     @NotBlank(message = "Owner name is required")
     private String ownerName;
 
+    // Same bounds as ExpenseInput: Quick Add creates stub expenses with amount 0
+    // and no category, and those stubs must remain editable.
     @NotNull(message = "Amount is required")
-    @DecimalMin(value = "0.01", message = "Amount must be greater than zero")
+    @DecimalMin(value = "0.00", message = "Amount must not be negative")
     private Double amount;
 
     private Double personalUseAmount;
@@ -34,7 +36,6 @@ public class ExpenseChangeRequest {
     @NotNull(message = "Date is required")
     private LocalDate date;
 
-    @NotBlank(message = "Category is required")
     private String category;
 
     public Expense convertToExpense(Long userId, ExpenseList expenseList) {
@@ -51,7 +52,9 @@ public class ExpenseChangeRequest {
         expense.setId(id);
         expense.setOwner(appUser);
         expense.setTitle(title);
-        expense.setCategory(category);
+        // The edit dialog sends "" for a missing category; store null so the
+        // stub state stays uniform and history diffs don't report null -> "".
+        expense.setCategory(category == null || category.isBlank() ? null : category);
 
         return expense;
     }

src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java

@@ -53,6 +53,54 @@ class ExpenseListControllerTest {
                 .andExpect(jsonPath("$.title").exists());
     }
 
+    // Quick Add creates stub expenses (amount 0, no category); updating such a
+    // stub must be allowed so the user can fill in the details afterwards.
+    @Test
+    void updateExpense_quickAddStub_zeroAmountAndNoCategory_returns200() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L); owner.setUsername("alice");
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(owner);
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(owner);
+        when(expenseListService.updateExpense(eq(1L), any(Expense.class), eq(owner)))
+                .thenAnswer(inv -> inv.getArgument(1));
+
+        mockMvc.perform(put("/api/expenselist/1/update")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"id\":42,\"title\":\"Groceries\",\"ownerName\":\"alice\",\"amount\":0.0,"
+                        + "\"personalUseAmount\":0.0,\"otherPersonAmount\":0.0,\"date\":\"2026-07-06\"}"))
+                .andExpect(status().isOk());
+    }
+
+    @Test
+    void updateExpense_blankCategory_isStoredAsNull() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L); owner.setUsername("alice");
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(owner);
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(owner);
+        when(expenseListService.updateExpense(eq(1L), any(Expense.class), eq(owner)))
+                .thenAnswer(inv -> inv.getArgument(1));
+
+        mockMvc.perform(put("/api/expenselist/1/update")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"id\":42,\"title\":\"Groceries\",\"ownerName\":\"alice\",\"amount\":0.0,"
+                        + "\"personalUseAmount\":0.0,\"otherPersonAmount\":0.0,\"date\":\"2026-07-06\",\"category\":\"\"}"))
+                .andExpect(status().isOk());
+
+        org.mockito.ArgumentCaptor<Expense> captor = org.mockito.ArgumentCaptor.forClass(Expense.class);
+        org.mockito.Mockito.verify(expenseListService).updateExpense(eq(1L), captor.capture(), eq(owner));
+        org.assertj.core.api.Assertions.assertThat(captor.getValue().getCategory()).isNull();
+    }
+
+    @Test
+    void updateExpense_negativeAmount_returns400() throws Exception {
+        mockMvc.perform(put("/api/expenselist/1/update")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"id\":42,\"title\":\"Groceries\",\"ownerName\":\"alice\",\"amount\":-5.0,"
+                        + "\"personalUseAmount\":0.0,\"otherPersonAmount\":0.0,\"date\":\"2026-07-06\",\"category\":\"Food\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.amount").exists());
+    }
+
     @Test
     void addExpense_negativeAmount_returns400() throws Exception {
         mockMvc.perform(post("/api/expenselist/1/add")