Update Stripe checkout processing fee display to 3%

- Changed line item name from "Processing Fee (5%)" to "Processing Fee (3%)"
- This updates what customers see on the Stripe checkout page
- Backend constant was already updated to 0.03 in previous commit
- Frontend receipt page was already showing 3%
- All processing fee displays now consistently show 3%

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

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

server/routes/checkout.js

@@ -81,7 +81,7 @@ router.post('/create-session', async (req, res) => {
         price_data: {
           currency: 'usd',
           product_data: {
-            name: 'Processing Fee (5%)',
+            name: 'Processing Fee (3%)',
             description: 'Transaction processing fee',
           },
           unit_amount: processingFee, // Processing fee in cents