feat: add full billing address collection to Stripe payment forms (refs #55)

- Replace CardElement with PaymentElement + AddressElement in subscription forms
- Add AddressElement to donation forms for billing address collection
- Now collects: Name, Address Line 1/2, City, State, Postal Code, Country
- Card details: Card Number, Expiration, CVC
- Both desktop and mobile forms updated

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

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

frontend/src/features/subscription/mobile/SubscriptionMobileScreen.tsx

@@ -1,6 +1,7 @@
 import React, { useState } from 'react';
 import { Elements } from '@stripe/react-stripe-js';
 import { loadStripe } from '@stripe/stripe-js';
+import type { StripeElementsOptions } from '@stripe/stripe-js';
 import { format } from 'date-fns';
 import { GlassCard } from '../../../shared-minimal/components/mobile/GlassCard';
 import { MobileContainer } from '../../../shared-minimal/components/mobile/MobileContainer';
@@ -18,6 +19,12 @@ import type { BillingCycle, SubscriptionTier, SubscriptionPlan } from '../types/
 
 const stripePromise = loadStripe(import.meta.env.VITE_STRIPE_PUBLISHABLE_KEY || '');
 
+const paymentElementsOptions: StripeElementsOptions = {
+  mode: 'setup',
+  currency: 'usd',
+  paymentMethodCreation: 'manual',
+};
+
 interface MobileTierCardProps {
   plan: SubscriptionPlan;
   billingCycle: BillingCycle;
@@ -358,7 +365,7 @@ export const SubscriptionMobileScreen: React.FC = () => {
         onClose={() => !checkoutMutation.isPending && setShowPaymentDialog(false)}
         title={`Upgrade to ${selectedTier && PLANS.find((p) => p.tier === selectedTier)?.name}`}
       >
-        <Elements stripe={stripePromise}>
+        <Elements stripe={stripePromise} options={paymentElementsOptions}>
           <PaymentMethodForm
             onSubmit={handlePaymentSubmit}
             isLoading={checkoutMutation.isPending}