Fix state persistence: save state before navigation

Issue: Calculator inputs were not being saved before navigating to checkout
because navigation happened synchronously before useEffect could run.

Solution: Call saveState() directly in handleCalculate before navigation
in both TripCalculator and MobileCalculator components.

This ensures calculator inputs (fuel amount, distance, etc.) are properly
preserved when users click "Calculate Impact" and then cancel checkout.

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

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

src/components/MobileCalculator.tsx

@@ -82,6 +82,19 @@ export function MobileCalculator({ vesselData, onOffsetClick, onBack }: Props) {
 
   const handleCalculate = useCallback((e: React.FormEvent) => {
     e.preventDefault();
+
+    // Save state before setting results
+    saveState({
+      calculationType,
+      distance,
+      speed,
+      fuelRate,
+      fuelAmount,
+      fuelUnit,
+      customAmount,
+      offsetPercentage: 100,
+    });
+
     if (calculationType === 'distance') {
       const estimate = calculateTripCarbon(
         vesselData,
@@ -99,7 +112,7 @@ export function MobileCalculator({ vesselData, onOffsetClick, onBack }: Props) {
         co2Emissions
       });
     }
-  }, [calculationType, distance, speed, fuelRate, fuelAmount, fuelUnit, vesselData]);
+  }, [calculationType, distance, speed, fuelRate, fuelAmount, fuelUnit, customAmount, vesselData, saveState]);
 
 
   const handleCustomAmountChange = useCallback((e: React.ChangeEvent<HTMLInputElement>) => {

src/components/TripCalculator.tsx

@@ -66,6 +66,19 @@ export function TripCalculator({ vesselData, onOffsetClick }: Props) {
 
   const handleCalculate = useCallback((e: React.FormEvent) => {
     e.preventDefault();
+
+    // Save state before navigating
+    saveState({
+      calculationType,
+      distance,
+      speed,
+      fuelRate,
+      fuelAmount,
+      fuelUnit,
+      customAmount,
+      offsetPercentage: 100,
+    });
+
     if (calculationType === 'distance') {
       const estimate = calculateTripCarbon(
         vesselData,
@@ -80,7 +93,7 @@ export function TripCalculator({ vesselData, onOffsetClick }: Props) {
       // Immediately navigate to projects page without showing results
       onOffsetClick?.(co2Emissions);
     }
-  }, [calculationType, distance, speed, fuelRate, fuelAmount, fuelUnit, vesselData, onOffsetClick]);
+  }, [calculationType, distance, speed, fuelRate, fuelAmount, fuelUnit, customAmount, vesselData, onOffsetClick, saveState]);
 
 
   const handleCustomAmountChange = useCallback((e: React.ChangeEvent<HTMLInputElement>) => {