feat(shutdown): confirm before stopping the daemon mid witness-cache rebuild

Stopping dragonxd while it's rebuilding the Sapling witness cache discards the
in-progress work — BuildWitnessCache aborts on shutdown without persisting — so
the next launch redoes a multi-minute rebuild (the "Activating best chain…" hang).
This bites especially with stop_external_daemon enabled, where wallet exit sends
the node a stop.

beginShutdown() now defers when it would StopDaemon while a rebuild is active and
shows a confirm modal: "Keep node running & quit" (DisconnectOnly — leaves it up
to finish), "Stop anyway & quit", or "Cancel". Rebuild detection reads the
debug.log tail markers (Cleared witness data / Setting Initial Sapling Witness /
Reading blocks for witness rebuild, vs. the "rebuilt … in …ms" / abort lines).
The gate lives entirely in beginShutdown()/render() — no SDL event-loop changes.

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

src/app.h

@@ -182,6 +182,14 @@ public:
      */
     std::vector<std::string> tailDaemonDebugLog(int maxLines) const;
 
+    // True when the daemon's debug.log shows an in-progress Sapling witness-cache rebuild (best-effort
+    // heuristic). Stopping the daemon during one discards it and forces a multi-minute redo next launch.
+    bool daemonWitnessRebuildActive() const;
+    // Whether beginShutdown() should pause and confirm before stopping the daemon (rebuild in progress).
+    bool shouldConfirmDaemonStop() const;
+    // The "node is rebuilding — stop anyway / keep running / cancel" modal, rendered from render().
+    void renderDaemonStopConfirm();
+
     /**
      * @brief Render loading overlay in content area while daemon is starting/syncing
      * @param contentH Height of the content area child window
@@ -911,6 +919,11 @@ private:
     bool address_list_dirty_ = false;   // P8: dedup rebuildAddressList
     GuardedStatus shutdown_status_;   // thread-safe: written by the shutdown thread, read by the UI (M-05)
     std::thread shutdown_thread_;
+    // Confirm-before-stopping-daemon-mid-witness-rebuild guard (see beginShutdown / renderDaemonStopConfirm)
+    bool pending_shutdown_confirm_ = false;       // a quit is deferred, waiting to open the confirm modal
+    bool daemon_stop_confirm_open_ = false;       // the confirm modal is currently showing
+    bool shutdown_confirmed_ = false;             // user chose to proceed — bypass the guard on re-entry
+    bool shutdown_keep_daemon_override_ = false;  // user chose "keep node running" for this shutdown only
     float shutdown_timer_ = 0.0f;
     bool force_quit_confirm_ = false;
     std::chrono::steady_clock::time_point shutdown_start_time_;