feat(sdk,wallet): real-time wallet activity feed via SSE

[collinsezedike]

Closes #320

Summary

• Adds sdk/src/events.ts: exports mapHorizonOpToEvent (maps raw Horizon ops to typed ActivityEvent records) and createActivityFeed (opens a Horizon SSE stream with exponential-backoff reconnect, resuming from the last seen paging token to prevent gaps or duplicates)
• Adds frontend/wallet/lib/activityFeed.ts: module-level pub/sub store (initActivityFeed, hydrateActivityFeed, subscribeActivityFeed, useActivityFeed) that wraps the SDK feed and converts events to the dashboard's TxRecord shape
• Updates frontend/wallet/app/dashboard/page.tsx: swaps manual transactions state for useActivityFeed(), seeds the feed with the initial history fetch, and starts the live stream once the fee-payer key is known
• Adds sdk/src/__tests__/events.test.ts: 15 unit tests covering the event mapper (all op types, edge cases) and the reconnect state machine (backoff timing, cursor resume, cap at 30 s, stop cancels reconnect)

Test plan

• Run cd sdk && npm test - all events tests pass
• Open the wallet dashboard and send/receive XLM on testnet - new transactions appear in the activity feed without a manual refresh
• Disconnect network briefly and reconnect - the stream recovers automatically and no events are duplicated
• Navigate away from dashboard and back - activity list renders instantly from module cache, then stream continues

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