feat(billing): update frontend for polar migration

Update billing types, remove invoice preview, replace Stripe invoice
display with Polar orders, update tax ID from array to single object,
remove upcoming invoice amount display.

components/PricingSection.tsx

@@ -109,7 +109,7 @@ export default function PricingSection() {
   const [loadingPlan, setLoadingPlan] = useState<string | null>(null)
   const { user } = useAuth()
 
-  // * Show toast when redirected from Stripe Checkout with canceled=true
+  // * Show toast when redirected from Polar Checkout with canceled=true
   useEffect(() => {
     if (searchParams.get('canceled') === 'true') {
       toast.info('Checkout was canceled. You can try again whenever you’re ready.')
@@ -196,7 +196,7 @@ export default function PricingSection() {
         limit,
       })
 
-      // 3. Redirect to Stripe Checkout
+      // 3. Redirect to Polar Checkout
       if (url) {
         window.location.href = url
       } else {