Adopt trustflow-sdk and consolidate hand-rolled wallet/contract integration

[meshackyaro]

Description

trustflow-sdk is not listed as a dependency anywhere in this repo's package.json. Instead, the frontend hand-rolls its own wallet integration directly against @stellar/freighter-api (hooks/useAccount.ts, components/atoms/wallet-button/) and defines its own env-var-based contract constants (shared/contracts.ts) with no code that actually invokes the SDK's contract/escrow clients. This means every piece of Soroban integration work (see the transaction-flow spike, #79) would either duplicate what the SDK already has, or need to be built twice.

Component

Frontend

Difficulty

🔴 Hard

Tasks

• Add @trustflow/sdk as a dependency once its wallet/escrow exports are usable (coordinate with the SDK repo's hook-export fix)
• Replace the direct @stellar/freighter-api usage in useAccount.ts with the SDK's wallet connection helpers, preserving current UI behavior
• Replace shared/contracts.ts's standalone env constants with the SDK's ClientConfig, keeping NEXT_PUBLIC_-prefixed env vars as the source of truth
• Audit for behavior drift between the current hand-rolled polling (setInterval(syncAccount, 2000)) and whatever connection model the SDK provides, and reconcile
• Update the README's stated architecture to reflect the SDK now being the integration boundary

Acceptance Criteria

• Wallet connect/disconnect/switch-account flows work identically to today from a user's perspective, now backed by the SDK
• No direct @stellar/freighter-api imports remain outside the SDK dependency boundary
• shared/contracts.ts sources its config from the SDK's types rather than ad hoc local constants
• Existing pages/components using useAccount require no API changes (or changes are scoped and called out explicitly)

Estimated Time

2-3 days