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/util/i18n.cpp

@@ -568,6 +568,7 @@ void I18n::loadBuiltinEnglish()
         strings_["tx_loading_scanning_shielded"] = "Scanning shielded history (%d addresses)";
         strings_["tx_loading_refreshing_cached"] = "Refreshing wallet history (%d cached)";
         strings_["tx_loading_fetching_transparent"] = "Fetching transparent history";
+        strings_["tx_loading_history_progress"] = "Loading older history (%d%%)";
         strings_["no_matching"] = "No matching transactions";
         strings_["transaction_id"] = "TRANSACTION ID";
         strings_["search_placeholder"] = "Search...";