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.h

@@ -575,6 +575,10 @@ private:
     static constexpr int MAX_VIEWTX_PER_CYCLE = 25; // cap z_viewtransaction calls per refresh
     std::size_t shielded_history_scan_cursor_ = 0;
     bool shielded_history_scan_pending_ = false;
+    // False until the first full shielded-history scan finishes. Drives the History tab's
+    // "Loading older history…" progress so the user knows transactions are still streaming in
+    // after the first batch appears; goes quiet for the routine per-block re-scans afterward.
+    bool initial_history_scan_complete_ = false;
     std::unordered_map<std::string, int> shielded_history_scan_heights_;
 
     // P4b: z_viewtransaction result cache — avoids re-calling the RPC for