[Frontend] Real-time escrow updates via Stellar event streaming

[Gbangbolaoluwagbemiga]

Problem

The UI polls for contract state every 15 seconds. This means ~15s lag between a blockchain event and the UI reflecting it. Polling is also wasteful.

Solution

Subscribe to Horizon's Server-Sent Events for contract events (once issue #5 lands).

What needs to be done

1. `src/hooks/useEscrowEvents.ts` — subscribe to Horizon SSE stream, parse XDR events, return via callback
2. Wire into pages:
   • `DashboardPage` — refresh on `EscrowCreated` / `MilestoneApproved`
   • `ApprovalsPage` — refresh on `MilestoneSubmitted`
   • `MessagesPage` — replace `setInterval` with event stream
3. Reconnect with exponential backoff
4. Fallback to polling if Horizon SSE unavailable

Acceptance criteria

• Milestone approval reflects in UI within 2s of on-chain confirmation
• No `setInterval` polling for contract state
• Reconnect logic with backoff
• Works on local, testnet, mainnet Horizon

Depends on issue #5 (contract events)