feat: add SDK playground, unit tests, WalletConnect experiment, and sender dashboard

[blegodwin]

Summary

Frontend improvements for the bridgelet monorepo: SDK playground, component tests, WalletConnect exploration, and sender dashboard.

Changes

SDK Playground Page (#151)

• Interactive /sandbox/playground page where developers can invoke all BridgeletClient endpoints (getClaimDetails, createPaymentIntent, redeemClaim) from a UI
• Select endpoint, fill parameters, execute, and inspect raw JSON response/error
• Dev-only (guarded by NODE_ENV check)

Unit Tests (#143)

• RTL tests for ClaimStatusCard, WalletAddressInput, ToastNotification, SkeletonLoader covering all states and keyboard interactions

WalletConnect Experiment (#147)

• Exploration of Stellar WalletConnect support documented for send and claim flows

Sender Dashboard (#153)

• Sandbox page showing real-time claim status (pending, claimed, expired) with polling

Closes #143
Closes #147
Closes #151
Closes #153

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