docs: Add comprehensive integration guide to README

[Copilot]

The README lacked coverage of server-side setup, full customization options, the PaymentWidget component, and environment requirements — leaving integrators without a complete reference.

Changes

Requirements

• Runtime requirements table (Node.js ≥18, RN ≥0.71, iOS ≥13, Android API ≥21)
• Peer dependencies with purpose descriptions (react-native-svg, react-native-inappbrowser-reborn, react-native-webview)
• Hyperswitch account prerequisites (publishable vs. secret key usage)

Server-side: Creating a Payment Intent

• API endpoint, headers, and request body reference (sandbox + production URLs)
• Node.js/Express and Python/Flask server examples
• Clarifies client_secret → clientSecret camelCase transformation

Customizations

• Full appearance API: theme (Default/Light/Dark/Minimal/FlatMinimal), layout (tabs/accordion/spacedAccordion), light/dark color overrides (all 13 color properties), shapes, primary button, font scaling, Google Pay/Apple Pay button config, locale
• Complete end-to-end customization example

Present Sheet

• presentPaymentSheet() usage with full result type reference
• paymentResult.status values ("succeeded" / "cancelled" / "Failed")

Payment Widget

• PaymentWidget props reference and onPaymentResult callback fields
• Contrasts widget (options.clientSecret + callback) vs. sheet (initPaymentSession + promise) patterns

<PaymentWidget
  widgetId="checkout-widget"
  widgetType="PAYMENT_SHEET"
  options={{ clientSecret, appearance: { theme: 'Light' } }}
  onPaymentResult={(result) => {
    if (result.errorMessage) {
      setStatus(`Payment failed: ${result.errorMessage}`);
    } else {
      setStatus(result.status); // "succeeded" | "cancelled" | "Failed"
    }
  }}
  style={{ width: '100%', height: 600 }}
/>

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