Add Buy G$ widget package with Onramper embed, SDK flow wiring, and QA fixtures

[Copilot]

This PR implements the planned “Buy G$” GoodWidget by introducing a dedicated widget package that embeds Onramper and wires the BuyGD SDK lifecycle into the GoodWidget runtime model. It covers the required widget states and adds deterministic Storybook/Playwright fixtures for review.

• Widget package: @goodwidget/buy-gd-widget

  • Added a new publishable widget package under packages/buy-gd-widget.
  • Introduced runtime contract types for Buy G$ state/action modeling (idle, loading, onramper, transaction_pending, success, error, no_wallet).
  • Exposed integration metadata and custom-element entrypoints (element, register) for host embedding.

• Buy flow integration

  • Added BuyGdWidget container + BuyGdView state UI composition.
  • Embedded Onramper via iframe with configurable query params (apiKey, default fiat/crypto, theme, allowed cryptos).
  • Wired wallet context from GoodWidgetProvider and BuyGD transaction lifecycle via @gooddollar/web3sdk-v2 (useBuyGd) and @usedapp/core provider boundary.
  • Added pending/success/error transitions with retry/refresh paths and host callbacks (onBuySuccess, onBuyError).

• Storybook QA fixtures

  • Added deterministic fixture states for all required runtime states.
  • Added QA/BuyGdWidget/Runtime Fixtures stories, including mock-driven states and injected-wallet manual path.

• Playwright smoke coverage

  • Added tests/widgets/buy-gd-widget/states.spec.ts covering all required states and screenshot outputs under tests/widgets/buy-gd-widget/test-results.

<BuyGdWidget
  provider={provider}
  onramperApiKey={apiKey}
  onramperOnlyCryptos="USDC,cUSD"
  onramperDefaultFiat="USD"
  onramperDefaultCrypto="cUSD"
/>