Add PayPal payment provider integration

[rogersole]

Summary

• Adds PayPal as a payment provider following the same architecture as the existing Paddle integration
• New PayPalService (src/paypal/paypal-service.ts) handles SDK initialization via @paypal/paypal-js, checkout start, PayPal button rendering, and operation status polling
• API key detection for ppl_ prefix keys, with routing in purchase() to performPayPalPurchase()
• Svelte UI components (paypal-purchases-ui.svelte, paypal-purchases-ui-inner.svelte) for loading/buttons/success/error pages
• PayPalCheckoutStartResponse type with paypal_billing_params (client_id, order_id, is_sandbox)
• BackendInvalidPayPalAPIKey error code and InvalidPayPalAPIKeyError flow error code
• 22 unit tests covering initialization, checkout start, purchase flow, cancellation, and polling

How it works

1. User calls purchase() with a ppl_ API key → routes to performPayPalPurchase()
2. PayPalService.startCheckout() calls backend's /checkout/start → gets order ID + client ID
3. PayPal JS SDK is loaded and buttons are rendered in the UI
4. User clicks PayPal button → completes payment in PayPal popup
5. On approval, polls /checkout/{operationSessionId} for completion → returns result

Test plan

• PayPal service unit tests pass (22 tests)
• Existing Paddle service tests still pass (27 tests)
• Existing main tests still pass (73 tests)
• TypeScript compilation passes
• Manual E2E testing with PayPal sandbox (requires backend support)
• Verify PayPal button rendering in different browsers

Note: This is a draft PR. Backend PayPal support is required for full E2E testing.

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