perf(migrate): gate the seed-migration confirming balance scan

While the seed migration sits on the Confirming gate it polled the
sweep's confirmations and a full wallet balance together on every pass.
That balance call is the expensive one, and it only matters once the
sweep is mined and we do not yet have a post-mine figure, yet it ran
every poll even mid-sync when it cannot move the gate.

- The confirmation count is still read on every poll.
- The balance scan now runs only when it can actually advance the adopt
  gate: the sweep is mined, no post-mine balance has been recorded yet,
  and no sync is in progress. The manual Refresh button forces it
  regardless.
- The decision lives in a small pure helper, and the poller snapshots
  the main-thread state it needs rather than reading members off the
  worker thread.
- The Confirming screen now claims a leftover remainder (and offers the
  re-sweep button) only once the balance has actually been measured; while
  it is still unknown it keeps waiting instead of prompting on a sentinel.

Tests cover the gate helper across the forced, syncing, and mined cases.

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

src/services/seed_migration_poll.h

@@ -0,0 +1,20 @@
+// DragonX Wallet - ImGui Edition
+// Copyright 2024-2026 The Hush Developers
+// Released under the GPLv3
+
+#pragma once
+
+namespace dragonx {
+namespace services {
+
+// The expensive z_gettotalbalance only changes the seed-migration adopt gate once the sweep is mined and
+// we have no post-mine figure yet (priorRemaining < 0). A manual Refresh forces it; an active sync
+// suppresses the automatic scan (the Refresh button still overrides).
+inline bool migrationWantsBalanceScan(bool forceBalance, bool syncing, int confs, double priorRemaining)
+{
+    if (forceBalance) return true;
+    return !syncing && confs >= 1 && priorRemaining < 0.0;
+}
+
+} // namespace services
+} // namespace dragonx