fix(balance): stop polling the account endpoint once it 404s

[Danswar]

Problem

BalanceService.startSync installs a 10s Timer.periodic that GETs /v1/realunit/account/<address> to refresh the RealUnit balance. For a wallet that has no RealUnit account, that endpoint returns 404 "Account not found" on every tick — and updateBalance only acts on 200, so it silently keeps re-requesting forever (~6 req/min/wallet).

Server-side (api.dfx.swiss) this is a steady stream of 404s that, because the response body contains "error": "Not Found", gets logged and counted as errors — a recurring false "error spike" tracked back to this app.

Fix

Mark the account missing on the first 404 and skip the network call on subsequent ticks; startSync resets the flag so app resume / re-init / a post-onboarding restart re-checks (the account may exist by then).

• Gates the HTTP call rather than cancelling the timer — HomeBloc calls updateBalance(addr) (unawaited) immediately followed by startSync(addr), so cancelling the timer from inside updateBalance would race the just-started timer. A flag has no such race.
• 200 still updates normally; transient 5xx are unaffected — only a definitive 404 (account genuinely absent) stops the poll.

Tests

Adds two fakeAsync cases to balance_service_test.dart:

• after a 404, later ticks make no further network calls;
• a fresh startSync resets the flag and re-probes (covers onboarding/resume).

Note

I don't have a local Flutter toolchain, so I relied on the existing test patterns — please let CI (flutter analyze + flutter test) confirm. The change is confined to BalanceService.