PayPal integration into purchases-js as payment gateway

[rogersole]

Summary

• Adds PayPal as a payment gateway (like Stripe), not an external billing integration (like Paddle)
• PayPal availability is determined by the backend returning paypal_gateway_params in checkout/prepare and checkout/start responses — no separate API key prefix
• Uses a popup window for PayPal's approval flow, with backend operation status polling after the popup closes
• Aligns with backend changes in https://github.com/RevenueCat/khepri/pull/18770

Changes

New files:

• src/paypal/paypal-service.ts — Popup + polling logic for PayPal approval flow
• src/tests/paypal/paypal-service.test.ts — Unit tests for PayPalService

Modified files:

• src/networking/responses/checkout-start-response.ts — Added PayPalGatewayParams and paypal_gateway_params field to WebBillingCheckoutStartResponse
• src/networking/responses/checkout-prepare-response.ts — Updated CheckoutPreparePayPalGatewayParams from { client_access_token } to { is_sandbox }
• src/helpers/purchase-operation-helper.ts — Exposed getBackend() and currentOperationSessionId for PayPal flow
• src/ui/purchases-ui.svelte — Extracts and passes paypal_gateway_params from checkout start response
• src/ui/purchases-ui-inner.svelte — Passes paypalGatewayParams through to payment entry page
• src/ui/pages/payment-entry-page.svelte — Renders "Pay with PayPal" button when paypal_gateway_params is present
• src/helpers/api-key-helper.ts — Removed pypl_ API key prefix and isPayPalApiKey()
• src/main.ts — Removed separate PayPal routing and performPayPalPurchase()

Deleted files:

• src/ui/paypal-purchases-ui.svelte — Standalone PayPal UI (replaced by integration into existing checkout)
• src/ui/paypal-purchases-ui-inner.svelte — Standalone PayPal inner UI

Backend contract (from Khepri PR)

POST /checkout/start returns paypal_gateway_params as an optional field alongside existing Stripe fields:

{
  "operation_session_id": "...",
  "gateway_params": { ... },
  "stripe_billing_params": { ... },
  "paypal_gateway_params": {
    "order_id": "...",
    "approval_url": "https://www.paypal.com/checkoutnow?token=...",
    "is_sandbox": true
  }
}

POST /checkout/prepare returns:

{
  "stripe_gateway_params": { ... },
  "paypal_gateway_params": {
    "is_sandbox": true
  }
}

Test plan

• All 554 tests pass
• TypeScript typecheck passes (both tsconfig.json and tsconfig.test.json)
• Svelte check passes
• ESLint passes
• End-to-end test with Khepri PR in sandbox

🤖 Generated with Claude Code

[RevenueCat-Danger-Bot]

	1 Error
🚫	Label the PR using one of the change type labels. If you are not sure which label to use, choose pr:other.

Label	Description
pr:feat	A new feature. Use along with pr:breaking to force a major release.
pr:fix	A bug fix. Use along with pr:force_minor to force a minor release.
pr:other	Other changes. Catch-all for anything that doesn't fit the above categories. Releases that only contain this label will not be released. Use along with pr:force_patch, or pr:force_minor to force a patch or minor release.
pr:RevenueCatUI	Use along any other tag to mark a PR that only contains RevenueCatUI changes
pr:next_release	Preparing a new release
pr:dependencies	Updating a dependency
pr:phc_dependencies	Updating purchases-hybrid-common dependency
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