fix(ui): stop spurious "Blockchain rescan complete" toast after restoring from minimize

While minimized the main loop skips the frame body (SDL_Delay+continue), so App::update() —
which drains the daemon output via outputSince(daemon_output_offset_) — never runs and the
offset isn't advanced. On restore the whole accumulated backlog is parsed in one batch: a
background witness rebuild's progress lines (parsed as a rescan → state_.sync.rescanning=true)
AND its "rebuilt … in Xms" completion (parsed as finished) arrive together and fire
"Blockchain rescan complete" for a scan the user never initiated.

On WINDOW_RESTORED, discard the daemon-output backlog (advance the offset to the current end,
via App::skipDaemonOutputBacklog) before the resumed update parses it — so only new output is
parsed. Genuine user-initiated rescans still surface completion via the getrescaninfo monitor.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app.h

@@ -393,6 +393,10 @@ public:
 
     // Force refresh
     void refreshNow();
+    // Called on window restore: drop the daemon-output backlog that accumulated while minimized (the
+    // per-frame update loop was paused), so a background witness rebuild that started+finished during
+    // the minimize isn't parsed in one batch and mistaken for a completed rescan (spurious toast).
+    void skipDaemonOutputBacklog();
     void refreshMiningInfo();
     void refreshPeerInfo();
     void refreshMarketData();