perf(node): throttle RPC polling during sync so block download isn't slowed

The full-node wallet polled the daemon at the per-tab cadence regardless of sync state.
On the Peers/Network tab that meant getpeerinfo every 5s + core every 5s + a full
transaction scan on every new block — and blocks arrive fast during sync. Each of those
calls takes the daemon's cs_main lock, the same lock block connection needs, so the node
synced noticeably slower than on the lightweight Console tab (core 10s, no peer polling).

Make the refresh cadence sync-aware:
- RefreshScheduler::kSyncProfile {core 10s, transactions/addresses/peers disabled} is applied
  to ALL tabs while state_.sync.syncing, and reverts to the per-tab profile when sync ends.
  applyRefreshPolicy() picks the profile; update() re-applies it on the syncing<->synced
  transition. This suppresses getpeerinfo and the per-block tx scan during sync (that data is
  incomplete mid-sync anyway) — every tab now syncs as fast as Console.
- collectCoreRefreshResult(rpc, includeBalance): skip z_gettotalbalance (wallet lock + cs_main)
  while syncing; only getblockchaininfo runs, which is also what drives sync-progress detection.
  applyCoreRefreshResult already leaves the balance untouched when balanceOk is false.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.cpp

@@ -569,6 +569,12 @@ void App::update()
     // Wipe a secret (seed/private key) from the clipboard once its auto-clear delay elapses.
     pumpSecretClipboardClear();
 
+    // Re-apply the refresh cadence when sync starts/finishes: while syncing we throttle polling to
+    // a low-impact profile so RPC contention doesn't slow block download (see applyRefreshPolicy).
+    if (state_.sync.syncing != refresh_policy_syncing_) {
+        applyRefreshPolicy(current_page_);
+    }
+
     // Full-node RPC refreshes gate on ACTUAL RPC connectivity, not state_.connected. In lite
     // builds state_.connected is the lite-wallet "online" proxy (true when a wallet is open, to
     // enable the wallet UI), but there is no RPC daemon — so RPC polls (mining/balance/peers/txs)