refactor: optimize state initialization with lazy initializers

- Move billStates calculation into useState lazy initializer to prevent recalculation on re-renders
- Initialize allPaidMode based on billStates to reflect actual data state
- Use lazy initialization pattern for better performance
- Remove intermediate variables to simplify code

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

app/[locale]/home/multi-bill-edit/[year]/[month]/MultiBillEdit.tsx

@@ -26,18 +26,21 @@ export const MultiBillEdit: FC<MultiBillEditProps> = ({ locations, year, month }
     const t = useTranslations("multi-bill-edit");
     const router = useRouter();
 
-    // Initialize bill states from locations
-    const initialBillStates: BillState[] = locations.flatMap(location =>
-        location.bills.map(bill => ({
-            locationId: location._id,
-            billId: bill._id,
-            paid: bill.paid,
-        }))
+    const [isSaving, setIsSaving] = useState(false);
+
+    const [billStates, setBillStates] = useState<BillState[]>(() =>
+        locations.flatMap(location =>
+            location.bills.map(bill => ({
+                locationId: location._id,
+                billId: bill._id,
+                paid: bill.paid,
+            }))
+        )
     );
 
-    const [billStates, setBillStates] = useState<BillState[]>(initialBillStates);
-    const [isSaving, setIsSaving] = useState(false);
-    const [allPaidMode, setAllPaidMode] = useState(false);
+    const [allPaidMode, setAllPaidMode] = useState(() =>
+        billStates.length > 0 && billStates.every(bill => bill.paid)
+    );
 
     // Toggle individual bill paid status
     const handleBillToggle = (locationId: string, billId: string) => {