fix(sync): stop large-wallet balance polling from starving block connection

On a fully-shielded (ac_private=1) chain, z_gettotalbalance is O(mapWallet) and
holds the daemon's cs_main for its whole duration — ~20s on a ~5k-tx wallet. The
Overview refresh polled it every ~2s (twice: minconf 0 and 1), so cs_main was
held almost continuously, starving the single block-connection thread: the node
connected blocks only in the gaps between polls and could fall further behind
the tip than it caught up (observed live: gap growing 58→100 blocks while the
GUI was open, one core pegged on GetFilteredNotes, 22 idle, ~17 B/s download).

Two hardening changes on top of the existing "skip balance while syncing" guard:

- Hysteresis: keep the low-impact sync profile (and balance suppression) for a
  short settle window after catching up, so a large-wallet scan can't
  immediately re-starve connection and bounce the node back into syncing. Armed
  only on the syncing→caught-up edge, so a wallet synced from the start is never
  throttled at connect (effectivelySyncing()).

- Adaptive balance cadence: time each z_gettotalbalance scan and require the
  next poll to wait at least (cost / 10%), so balance scanning never occupies
  more than ~10% of wall-clock. Cheap wallets are unaffected (the tab's Core
  timer stays the cadence); a ~20s scan backs off to ~200s. Wallet mutations
  (send/shield) force the next poll through so the user's own action updates the
  balance immediately (balanceRefreshDue()).

getblockchaininfo keeps its normal cadence throughout, so sync progress stays
live. Build + test_phase4 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/services/network_refresh_service.cpp

@@ -3,6 +3,7 @@
 
 #include <algorithm>
 #include <cctype>
+#include <chrono>
 #include <cmath>
 #include <cstdlib>
 #include <map>
@@ -294,8 +295,13 @@ NetworkRefreshService::CoreRefreshResult NetworkRefreshService::collectCoreRefre
     json blockInfo;
     bool balanceOk = false;
     bool blockOk = false;
+    double balanceScanMs = 0.0;
 
     if (includeBalance) {
+        // z_gettotalbalance is O(mapWallet) and holds the daemon's cs_main for its whole duration —
+        // seconds on a large shielded wallet. Time it so the caller can throttle how often it polls
+        // (balanceRefreshDue()), keeping balance scans from starving block connection.
+        const auto balanceStart = std::chrono::steady_clock::now();
         try {   // DISPLAY total: minconf=0 — includes the user's own pending change so it doesn't crater
             totalBalance = rpc.call("z_gettotalbalance", json::array({0}));
             balanceOk = true;
@@ -305,6 +311,8 @@ NetworkRefreshService::CoreRefreshResult NetworkRefreshService::collectCoreRefre
         try {   // SPENDABLE total: minconf=1 (confirmed). If absent, spendable degrades to display in apply.
             spendableBalance = rpc.call("z_gettotalbalance", json::array({1}));
         } catch (...) {}
+        balanceScanMs = std::chrono::duration<double, std::milli>(
+            std::chrono::steady_clock::now() - balanceStart).count();
     }
 
     try {
@@ -314,7 +322,9 @@ NetworkRefreshService::CoreRefreshResult NetworkRefreshService::collectCoreRefre
         DEBUG_LOGF("BlockchainInfo error: %s\n", e.what());
     }
 
-    return parseCoreRefreshResult(totalBalance, spendableBalance, balanceOk, blockInfo, blockOk);
+    auto result = parseCoreRefreshResult(totalBalance, spendableBalance, balanceOk, blockInfo, blockOk);
+    result.balanceScanMs = balanceScanMs;
+    return result;
 }
 
 NetworkRefreshService::MiningRefreshResult NetworkRefreshService::parseMiningRefreshResult(