Implement QR code auto-fill and auto-navigation for calculator

- Update useQRDecoder hook to use Next.js useSearchParams() for SSR compatibility
- Add auto-navigation logic to TripCalculator that automatically calculates and navigates to offset page when QR data is present
- Add QR detection to CalculatorClient to conditionally render QRCalculatorLoader vs TripCalculator
- Support all three calculation types: fuel, distance, and custom amount
- Add 500ms delay before navigation to allow UI to render

This completes the QR code flow where scanning a QR code now automatically:
1. Pre-fills calculator form with data
2. Calculates carbon footprint
3. Navigates to offset purchase page

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

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

components/CalculatorClient.tsx

@@ -2,7 +2,9 @@
 
 import { useState } from 'react';
 import { TripCalculator } from '../src/components/TripCalculator';
+import { QRCalculatorLoader } from '../src/components/QRCalculatorLoader';
 import { OffsetOrder } from '../src/components/OffsetOrder';
+import { useHasQRData } from '../src/hooks/useQRDecoder';
 import type { VesselData } from '../src/types';
 
 // Sample vessel data (same as in old App.tsx)
@@ -16,6 +18,7 @@ const sampleVessel: VesselData = {
 };
 
 export function CalculatorClient() {
+  const hasQRData = useHasQRData(); // Check if URL contains QR code data
   const [showOffsetOrder, setShowOffsetOrder] = useState(false);
   const [offsetTons, setOffsetTons] = useState(0);
   const [monetaryAmount, setMonetaryAmount] = useState<number | undefined>();
@@ -59,10 +62,17 @@ export function CalculatorClient() {
       </div>
 
       <div className="flex flex-col items-center w-full max-w-4xl space-y-8">
-        <TripCalculator
-          vesselData={sampleVessel}
-          onOffsetClick={handleOffsetClick}
-        />
+        {hasQRData ? (
+          <QRCalculatorLoader
+            vesselData={sampleVessel}
+            onOffsetClick={handleOffsetClick}
+          />
+        ) : (
+          <TripCalculator
+            vesselData={sampleVessel}
+            onOffsetClick={handleOffsetClick}
+          />
+        )}
       </div>
     </div>
   );

src/components/TripCalculator.tsx

@@ -1,3 +1,5 @@
+'use client';
+
 import { useState, useCallback, useEffect } from 'react';
 import { Leaf, Droplet } from 'lucide-react';
 import { motion, AnimatePresence } from 'framer-motion';
@@ -75,6 +77,37 @@ export function TripCalculator({ vesselData, onOffsetClick, initialData }: Props
     initialData?.customAmount?.toString() || savedState?.customAmount || ''
   );
 
+  // Auto-navigate when QR data is present
+  useEffect(() => {
+    if (initialData && onOffsetClick) {
+      // Automatically calculate and navigate based on calculation type
+      if (initialData.calculationType === 'distance' && initialData.distance && initialData.speed && initialData.fuelRate) {
+        const estimate = calculateTripCarbon(
+          initialData.distance,
+          initialData.speed,
+          initialData.fuelRate
+        );
+        // Small delay to allow UI to render before navigating
+        setTimeout(() => {
+          onOffsetClick(estimate.co2Emissions);
+        }, 500);
+      } else if (initialData.calculationType === 'fuel' && initialData.fuelAmount && initialData.fuelUnit) {
+        const co2Emissions = calculateCarbonFromFuel(
+          initialData.fuelAmount,
+          initialData.fuelUnit === 'gallons'
+        );
+        setTimeout(() => {
+          onOffsetClick(co2Emissions);
+        }, 500);
+      } else if (initialData.calculationType === 'custom' && initialData.customAmount) {
+        // For custom amount, pass the monetary value directly
+        setTimeout(() => {
+          onOffsetClick(0, initialData.customAmount);
+        }, 500);
+      }
+    }
+  }, [initialData, onOffsetClick]);
+
   const handleCalculate = useCallback((e: React.FormEvent) => {
     e.preventDefault();
 

src/hooks/useQRDecoder.ts

@@ -3,7 +3,10 @@
  * Custom React hook for decoding QR calculator data from URL parameters
  */
 
+'use client';
+
 import { useState, useEffect } from 'react';
+import { useSearchParams } from 'next/navigation';
 import { QRCalculatorData } from '../types';
 import { extractQRDataFromUrl } from '../utils/qrDataEncoder';
 import { validateQRData } from '../utils/qrDataValidator';
@@ -49,6 +52,7 @@ export interface QRDecoderResult {
  * ```
  */
 export function useQRDecoder(): QRDecoderResult {
+  const searchParams = useSearchParams();
   const [data, setData] = useState<QRCalculatorData | null>(null);
   const [isLoading, setIsLoading] = useState(true);
   const [error, setError] = useState<string | null>(null);
@@ -57,11 +61,8 @@ export function useQRDecoder(): QRDecoderResult {
   useEffect(() => {
     const decodeQRData = () => {
       try {
-        // Get URL search params
-        const searchParams = new URLSearchParams(window.location.search);
-
         // Check if QR parameter exists
-        const qrParam = searchParams.get('qr');
+        const qrParam = searchParams?.get('qr');
         if (!qrParam) {
           setHasQRData(false);
           setIsLoading(false);
@@ -71,7 +72,9 @@ export function useQRDecoder(): QRDecoderResult {
         setHasQRData(true);
 
         // Extract and decode QR data
-        const decodedData = extractQRDataFromUrl(searchParams);
+        // Need to convert ReadonlyURLSearchParams to URLSearchParams for compatibility
+        const urlSearchParams = new URLSearchParams(searchParams?.toString());
+        const decodedData = extractQRDataFromUrl(urlSearchParams);
 
         if (!decodedData) {
           setError('Failed to decode QR data');
@@ -100,7 +103,7 @@ export function useQRDecoder(): QRDecoderResult {
     };
 
     decodeQRData();
-  }, []); // Run only once on mount
+  }, [searchParams]); // Re-run when search params change
 
   return {
     data,
@@ -128,12 +131,12 @@ export function useQRDecoder(): QRDecoderResult {
  * ```
  */
 export function useHasQRData(): boolean {
+  const searchParams = useSearchParams();
   const [hasQRData, setHasQRData] = useState(false);
 
   useEffect(() => {
-    const searchParams = new URLSearchParams(window.location.search);
-    setHasQRData(searchParams.has('qr'));
-  }, []);
+    setHasQRData(searchParams?.has('qr') || false);
+  }, [searchParams]);
 
   return hasQRData;
 }