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>

tests/test_phase4.cpp

@@ -44,6 +44,7 @@
 #include "services/wallet_scan_governor.h"
 #include "services/scan_policy.h"
 #include "services/connect_scan_gate.h"
+#include "services/seed_migration_poll.h"
 #include "services/chat_scan_policy.h"
 #include "services/mining_thread_commit.h"
 #include "services/network_refresh_service.h"
@@ -18292,6 +18293,17 @@ void testLiteWalletExistsCache()
     dragonx::test::g_liteFakeWalletExists = true;     // restore default for later tests
 }
 
+void testMigrationWantsBalanceScan()
+{
+    using dragonx::services::migrationWantsBalanceScan;
+    EXPECT_FALSE(migrationWantsBalanceScan(false, false, 0, -1.0));  // not mined
+    EXPECT_TRUE(migrationWantsBalanceScan(false, false, 1, -1.0));   // mined, no figure yet
+    EXPECT_FALSE(migrationWantsBalanceScan(false, false, 3, 0.0));   // already scanned
+    EXPECT_FALSE(migrationWantsBalanceScan(false, false, 3, 2.5));   // already scanned (remainder)
+    EXPECT_TRUE(migrationWantsBalanceScan(true, true, 0, 0.0));      // forced overrides sync / unmined
+    EXPECT_FALSE(migrationWantsBalanceScan(false, true, 1, -1.0));   // syncing defers the automatic scan
+}
+
 int main()
 {
     testConnectionConfig();
@@ -18348,6 +18360,7 @@ int main()
     testRefreshProfileSelection();
     testConnectionInfoSeeding();
     testConnectScanGate();
+    testMigrationWantsBalanceScan();
     testScanPolicy();
     testChatScanPolicy();
     testChooseChatPayer();