feat(history): show "Loading older history (N%)" during the initial bulk load

History streams in over many refresh cycles (the incremental shielded scan
walks every z-address), so the first batch appears long before the list is
complete — with no indication more is still coming. The existing loading banner
deliberately goes quiet once any rows are on screen.

Track whether the first full shielded scan has finished
(initial_history_scan_complete_) and, until it has, surface a progress percentage
(fraction of z-addresses scanned) in transactionRefreshProgressText() — which the
History tab already renders as its pulsing loading indicator. Goes quiet once the
first scan completes; routine per-block re-scans don't re-trigger it. Reset on a
full history invalidation (rescan / session reset) so it shows again on reload.

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

src/app_network.cpp

@@ -975,6 +975,7 @@ void App::invalidateShieldedHistoryScanProgress(bool persistCache)
 {
     shielded_history_scan_cursor_ = 0;
     shielded_history_scan_pending_ = false;
+    initial_history_scan_complete_ = false;  // a full re-scan is coming — show load progress again
     shielded_history_scan_heights_.clear();
     if (persistCache) storeTransactionHistoryCacheIfAvailable();
 }
@@ -1353,6 +1354,7 @@ void App::refreshTransactionData()
             storeTransactionHistoryCacheIfAvailable();
             shielded_history_scan_cursor_ = nextShieldedScanStartIndex;
             shielded_history_scan_pending_ = !shieldedScanComplete;
+            if (shieldedScanComplete) initial_history_scan_complete_ = true;
             transactions_dirty_ = !shieldedScanComplete;
             maybeFinishTransactionSendProgress();
         };