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.h

@@ -1155,7 +1155,7 @@ private:
     // opid the daemon no longer knows (stale) -> back to the dismissable Sweep gate (re-check balance).
     std::function<void(bool, const std::string&)> makeSweepCompletionCallback(bool resumed);
     void beginSweepToSeedWallet();       // z_mergetoaddress ["ANY_TADDR","ANY_ZADDR"] -> dest
-    void pollSweepStatus();              // Confirming step: poll sweep confirmations + legacy balance
+    void pollSweepStatus(bool forceBalance = false);  // Confirming step: confs always; balance only when it can move the gate
     void locateSweepTxid();              // recover a lost sweep txid (send to dest) -> Confirming gate
     void beginAdoptSeedWallet();         // stop daemon -> swap wallet.dat -> restart with -rescan
     void provisionChatIdentityFromSecret(std::string secret);