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/app.cpp

@@ -5015,7 +5015,10 @@ void App::renderSeedMigrationDialog()
         if (confirmed && legacyEmpty) {
             if (ui::material::TactileButton(TR("mig_adopt"), ImVec2(180 * dp, 0)))
                 beginAdoptSeedWallet();
-        } else if (confirmed && !legacyEmpty) {
+        } else if (confirmed && seed_migration_legacy_remaining_ >= 0.0 && !legacyEmpty) {
+            // Only claim a remainder once the legacy balance was actually measured; while a sweep is
+            // mined but the balance scan is still gated (node syncing), remaining stays at the -1.0
+            // sentinel, so fall through to the waiting state rather than offering a sweep on an unknown.
             ImGui::PushStyleColor(ImGuiCol_Text, ui::material::ReadableError());
             ImGui::TextWrapped("%s", TR("mig_remainder"));
             ImGui::PopStyleColor();
@@ -5026,7 +5029,7 @@ void App::renderSeedMigrationDialog()
         }
         ImGui::SameLine();
         if (ui::material::TactileButton(TR("refresh"), ImVec2(110 * dp, 0)))
-            pollSweepStatus();
+            pollSweepStatus(/*forceBalance=*/true);
         ImGui::SameLine();
         if (ui::material::TactileButton(TR("mig_later"), ImVec2(100 * dp, 0)))
             close();  // pending state + txid persist; resumes here next time