fix expenses grouping to parse occurred_at as DD-MM-YYYY strictly

occurred_at comes from the backend as a DD-MM-YYYY string, which
new Date() cannot parse (NaN), so most transactions fell into an
"unknown" bucket and month sorting was broken.

- add parseOccurredAt() that only accepts DD-MM-YYYY, day-first,
  and throws on any other format or invalid date
- monthKey() now throws instead of returning "unknown"
- add currentMonthKey() for the "this month" stat
- sort groupByMonth/summary by parsed timestamp

src/Expense/Expense.tsx

@@ -14,7 +14,7 @@ import { useResource, useAppContext, formatCurrency } from "../../react-openapi"
 import { PageHeader } from "../ui/PageHeader";
 import { EmptyState } from "../ui/EmptyState";
 import { ExpenseList } from "./ExpenseList";
-import { ExpenseItem, ExpenseFieldConfigs, isExpense, monthKey, monthLabel, resolveLogoUrl } from "./types";
+import { ExpenseItem, ExpenseFieldConfigs, isExpense, currentMonthKey, monthKey, monthLabel, parseOccurredAt, resolveLogoUrl } from "./types";
 
 const API_BASE = import.meta.env.VITE_API_BASE_URL;
 
@@ -94,7 +94,7 @@ export default function Expense() {
   const sorted = useMemo(
     () =>
       [...(items ?? [])].sort(
-        (a, b) => new Date(b.occurred_at ?? 0).getTime() - new Date(a.occurred_at ?? 0).getTime(),
+        (a, b) => parseOccurredAt(b.occurred_at).getTime() - parseOccurredAt(a.occurred_at).getTime(),
       ),
     [items],
   );
@@ -103,8 +103,7 @@ export default function Expense() {
     const currency = sorted.find((it) => it.account?.currency)?.account?.currency ?? "INR";
     const totalSpent = sorted.filter(isExpense).reduce((s, it) => s + (it.amount ?? 0), 0);
     const totalIncome = sorted.filter((it) => !isExpense(it)).reduce((s, it) => s + (it.amount ?? 0), 0);
-    const now = new Date();
-    const thisMonth = monthKey(now.toISOString());
+    const thisMonth = currentMonthKey();
     const monthItems = sorted.filter((it) => monthKey(it.occurred_at) === thisMonth);
     const monthTotal = monthItems.filter(isExpense).reduce((s, it) => s + (it.amount ?? 0), 0);
     return { currency, totalSpent, totalIncome, thisMonth: monthLabel(thisMonth), monthItems, monthTotal };

src/Expense/ExpenseList.tsx

@@ -9,7 +9,7 @@ import {
 import ExpandMoreIcon from "@mui/icons-material/ExpandMore";
 import { ListCellRenderer, CurrencyField, applyDisplayFormat, formatCurrency } from "../../react-openapi";
 import type { ExpenseItem, ExpenseFieldConfigs } from "./types";
-import { monthKey, monthLabel } from "./types";
+import { monthKey, monthLabel, parseOccurredAt } from "./types";
 import { ExpenseDetail } from "./ExpenseDetail";
 
 interface GroupedMonth {
@@ -30,7 +30,7 @@ function groupByMonth(items: ExpenseItem[]): GroupedMonth[] {
   return [...map.entries()]
     .map(([key, list]) => {
       const sorted = [...list].sort(
-        (a, b) => new Date(b.occurred_at ?? 0).getTime() - new Date(a.occurred_at ?? 0).getTime(),
+        (a, b) => parseOccurredAt(b.occurred_at).getTime() - parseOccurredAt(a.occurred_at).getTime(),
       );
       const currency = sorted.find((it) => it.account?.currency)?.account?.currency ?? "INR";
       const total = sorted.reduce((sum, it) => sum + (it.amount ?? 0), 0);

src/Expense/types.ts

@@ -36,13 +36,34 @@ export function resolveLogoUrl(logo?: string, base?: string): string | undefined
   return logo;
 }
 
-export function monthKey(iso?: string): string {
-  if (!iso) return "unknown";
-  const d = new Date(iso);
-  if (Number.isNaN(d.getTime())) return "unknown";
+export function parseOccurredAt(value?: string): Date {
+  const m = value?.match(/^(\d{2})-(\d{2})-(\d{4})$/);
+  if (!m) {
+    throw new Error(`Expense occurred_at is not DD-MM-YYYY: ${value}`);
+  }
+  const [, dd, mm, yyyy] = m;
+  const d = new Date(Number(yyyy), Number(mm) - 1, Number(dd));
+  if (
+    Number.isNaN(d.getTime()) ||
+    d.getDate() !== Number(dd) ||
+    d.getMonth() !== Number(mm) - 1 ||
+    d.getFullYear() !== Number(yyyy)
+  ) {
+    throw new Error(`Invalid expense occurred_at date: ${value}`);
+  }
+  return d;
+}
+
+export function monthKey(value?: string): string {
+  const d = parseOccurredAt(value);
   return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}`;
 }
 
+export function currentMonthKey(): string {
+  const now = new Date();
+  return `${now.getFullYear()}-${String(now.getMonth() + 1).padStart(2, "0")}`;
+}
+
 export function monthLabel(key: string): string {
   const [y, m] = key.split("-").map(Number);
   if (!y || !m) return key;