fix(walletapi): simplify sync loop to zero-SCID-only polling

[moralpriest]

Simplify the sync loop to only sync the native DERO (zero SCID) and remove the test_connectivity() gate from the loop condition.

Problem

The previous sync loop made RPC calls for every known token SCID on each iteration. This added overhead and created a concurrent map access issue when SyncHistory ran in the background (see PR #26). Additionally, the loop condition used test_connectivity() which made redundant RPC calls every 3 seconds.

Changes

• Remove test_connectivity() from the loop condition; IsDaemonOnline() is sufficient
• Replace EntriesNative iteration with zero-SCID-only sync
• Token balances sync on demand when wallet API callers request them

Notes

• This prevents concurrent map iteration and write panic when SyncHistory runs as a goroutine
• Balance queries from the wallet API trigger per-SCID sync on demand
• Reduces per-iteration RPC calls from N (all token SCIDs) to 1 (native DERO only)

[Dirtybird99]

The !IsDaemonOnline() guard flip is a genuine improvement (the old code fell through to a failing sync attempt when the socket was nil), and Sync_Wallet_Memory_With_Daemon() is exactly internal(zerohash), so the registered-no-entries path is unchanged. Three qualifications:

1. "Token balances sync on demand when wallet API callers request them" holds for the wallet JSON-RPC server (rpc_getbalance.go syncs the SCID first) but not for library consumers calling Get_Balance_scid()/Get_Balance() directly — those return the stored map value with no sync, so embedded-walletapi UIs will show stale token balances indefinitely, and token history stops updating outside RPC GetBalance calls. Deserves a loud release-note callout.
2. "Prevents concurrent map iteration and write panic" — this removes the sync_loop range, but the second race from #26 remains: Save_Wallet()'s json.Marshal(w.account) iterates EntriesNative unlocked while a background SyncHistory writes it (details on #26).
3. For unregistered wallets, daemon_height now freezes after connect (details and a suggested fix on #29).

Also worth stating explicitly in the description: #26 must not merge before this PR.