perf(net): hold wallet scans at connect, in the background and on tab entry

- Connect gate: after the node starts answering, wallet scans wait until the
  lock state is known and the node knows its tip (or the wallet has fewer
  than 500 transactions), at most 20 s. A node already behind latches the
  sync profile from the connect's getinfo. Meanwhile a list-only pass keeps
  the address list and the cached history up, and the session's first
  balance and per-address balances are read once even while catching up,
  so the wallet shows its last-synced values instead of 0.
- Background: minimized, hidden, or covered without focus (3 s debounce,
  any input cancels) polls only the chain state every 15 s. Wallet scans,
  the mining poll, price, nudges and auto-lock interaction tracking pause;
  auto-lock now fires while minimized. Leaving a held profile marks every
  timer due again.
- Tab entry marks the timers due instead of forcing scans; a stale list or
  a never-loaded history still loads on entry.
- The scan entry points own the hard gates, failed enumerations keep the
  previous lists behind a retry hold, and failed balance calls never
  overwrite known balances with 0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/services/refresh_scheduler.cpp

@@ -26,6 +26,18 @@ RefreshScheduler::Intervals RefreshScheduler::intervalsForPage(ui::NavPage page)
     }
 }
 
+RefreshScheduler::Intervals RefreshScheduler::selectIntervals(ui::NavPage page, bool syncing, bool background)
+{
+    if (background) return kBackgroundProfile;
+    if (syncing) return kSyncProfile;
+    return intervalsForPage(page);
+}
+
+static_assert(RefreshScheduler::kSyncProfile.core * 2.0f <= RefreshScheduler::kSyncSettleSeconds,
+              "the sync settle window must outlast two Core polls");
+static_assert(RefreshScheduler::kBackgroundProfile.core * 2.0f <= RefreshScheduler::kSyncSettleSeconds,
+              "the sync settle window must outlast two Core polls");
+
 void RefreshScheduler::applyPage(ui::NavPage page)
 {
     setIntervals(intervalsForPage(page));