fix(arkade): dispose unused ContractWatcher to stop log flood

[Danswar]

Problem

dfx-api logs are flooded by the Arkade integration on both dev and prod (~1.6k errors/hr combined), in two forms:

• Subscription error: ReferenceError: EventSource is not defined
• ContractWatcher connection failed: ... {"code":13,"message":"subscription not found: <uuid>"}

Wallet.create (@arkade-os/sdk) starts a background ContractWatcher that streams live VTXO/contract events over SSE. ArkadeClient never uses those events — it polls for state (getTransaction → finalizePendingTxs/getVtxos). So the watcher is pure dead weight, and it's broken in our runtime:

• Our image is Node 20 (node:20-alpine), which has no global EventSource (added in Node 22) → the SSE listen loop throws ReferenceError.
• The SDK's resubscribe path (0.4.36) reuses a stale subscription id and its stale-detection regex (/subscription\s+\S+\s+not\s+found/i) doesn't match the server's subscription not found: <id> message → it loops forever on the dead id.

Net effect is log noise only — no broken functionality, since ArkadeClient doesn't consume watcher events — but it drowns the logs and masks real errors.

Fix

Stop the watcher right after wallet creation via the SDK's public ContractManager.dispose(). Disposal sets isWatching=false, so connect()/scheduleReconnect() short-circuit and the loop ends. It's wrapped best-effort (never fatal) so a transient indexer hiccup can't break wallet init.

All wallet operations ArkadeClient actually uses — getAddress, getBalance, getVtxos, sendBitcoin, finalizePendingTxs — are unaffected (dispose() only stops the live SSE watch, not the manager's data access).

Test

• arkade-client.spec.ts + arkade.service.spec.ts: 30/30 pass (mocks updated with getContractManager).
• nest build (tsc) green against @arkade-os/sdk@0.4.36; eslint + prettier clean.

Verify after merge

On dfxdev/dfxprd dfx-api: EventSource is not defined and subscription not found drain to ~0.