feat: wire sse_client_cpp_core for live updates from /api/boards/issues/stream

panels.h

@@ -5,10 +5,15 @@
 #pragma once
 
 #include "data.h"
+#include <mutex>
+#include <string>
 
 namespace kanban_cpp {
 
 // Shared app state passed to every panel. Owned by main.cpp.
+// `mu` guards cards/columns/backend_ok/last_refresh_*/sse_status — refresh_data
+// corre en thread aparte y el SSE callback tambien lo hace, ambos escriben a
+// traves del mismo mutex.
 struct AppState {
     ClientConfig cfg;
     std::vector<Card>    cards;
@@ -16,6 +21,9 @@ struct AppState {
     std::string last_refresh_error;
     int64_t     last_refresh_ts = 0;
     bool        backend_ok = false;
+    // SSE live status: "connecting" | "connected" | "disconnected" | "error: <msg>"
+    std::string sse_status = "connecting";
+    std::mutex  mu;
 };
 
 void draw_board     (AppState& s, bool* p_open);