fix(history): let the shielded scan complete + unstick send-progress on many-z-addr wallets

Two issues shared one root cause: the shielded-receive scan marked each z-address "scanned
at the EXACT current tip," but a new block (~36s on DRGX) advances the tip and invalidates
every prior per-address scan. A wallet with more z-addresses than one refresh cycle can
scan therefore never reached "all scanned at tip" — so shieldedScanComplete stayed false
and transactions_dirty_ stayed true forever, which (a) kept the history-refresh banner lit
and the full rescan churning every cycle, and (b) blocked maybeFinishTransactionSendProgress
(it waited on transactions_dirty_), leaving the send-progress indicator stuck on.

Fix 1 — completion tolerance. Add TransactionRefreshSnapshot::shieldedScanTipTolerance: an
address counts as fresh if its last scan is within N blocks of the tip (0 = old strict
behavior, so existing tests are unchanged). The app scales N with the z-address count
(2 + count/96, capped at 50), so a multi-block pass can COMPLETE before its earliest scan
goes stale. This also throttles full rescans to ~N blocks instead of every block —
transactions_dirty_ clears, the banner stops, and CPU/RPC churn drops. Already-fresh
addresses are skipped, so the per-block cost falls back to just the (cheap) transparent
listtransactions.

Fix 2 — send-progress gate. maybeFinishTransactionSendProgress() no longer waits on the
transaction history scan (transactions_dirty_ / Transactions job): the sent tx is already
shown via the optimistic pending insert, and the spend is reflected once the balance
refresh lands, so it now finishes on the address/balance signal alone.

Test: a tolerant snapshot skips recently-scanned addresses (shieldedAddressesScanned == 0,
shieldedScanComplete) while a strict one re-scans them.

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

src/services/network_refresh_service.cpp

@@ -822,10 +822,14 @@ NetworkRefreshService::TransactionRefreshResult NetworkRefreshService::collectTr
         DEBUG_LOGF("listtransactions error: %s\n", e.what());
     }
 
+    // An address counts as "scanned at tip" if its last scan is within shieldedScanTipTolerance
+    // blocks of the current tip. Tolerance 0 means strict (exact tip); a small tolerance lets a
+    // multi-cycle pass over many addresses complete even as the tip advances during it.
+    const int scanStaleThreshold = currentBlockHeight - std::max(0, snapshot.shieldedScanTipTolerance);
     auto scannedAtTip = [&](const std::string& address) {
         if (currentBlockHeight < 0) return false;
         auto it = result.shieldedScanHeights.find(address);
-        return it != result.shieldedScanHeights.end() && it->second >= currentBlockHeight;
+        return it != result.shieldedScanHeights.end() && it->second >= scanStaleThreshold;
     };
 
     std::size_t shieldedStart = snapshot.shieldedScanStartIndex;

src/services/network_refresh_service.h

@@ -178,6 +178,13 @@ public:
         std::unordered_map<std::string, int> shieldedScanHeights;
         std::size_t shieldedScanStartIndex = 0;
         std::size_t maxShieldedReceiveScans = 0;
+        // How many blocks the tip may advance past an address's last scan before it counts as stale
+        // and needs re-scanning. 0 = strict (must be scanned at the exact current tip). A small
+        // tolerance lets a multi-cycle pass over many shielded addresses COMPLETE even though new
+        // blocks arrive mid-pass — otherwise the "scanned at tip" bar moves every block and the scan
+        // (and the transactions_dirty_ flag it drives) never finishes. It also naturally throttles
+        // full rescans to roughly once per `tolerance` blocks.
+        int shieldedScanTipTolerance = 0;
     };
 
     struct TransactionRefreshResult {