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

panel_calendar.cpp

@@ -8,6 +8,7 @@
 
 #include <imgui.h>
 #include <ctime>
+#include <mutex>
 
 namespace kanban_cpp {
 
@@ -32,6 +33,12 @@ void draw_calendar(AppState& s, bool* p_open) {
     ImGui::TextDisabled("(MVP estatico — TODO: navegacion + filtros)");
     ImGui::Separator();
 
+    std::vector<Card> cards_snap;
+    {
+        std::lock_guard<std::mutex> lock(s.mu);
+        cards_snap = s.cards;
+    }
+
     // First day of current month + days in month
     std::tm tm_first = tm_now;
     tm_first.tm_mday = 1;
@@ -68,7 +75,7 @@ void draw_calendar(AppState& s, bool* p_open) {
                     ImGui::Text("%d", day);
                     // Count cards whose due_date falls in this day.
                     int hits = 0;
-                    for (const auto& card : s.cards) {
+                    for (const auto& card : cards_snap) {
                         if (card.due_date == 0) continue;
                         std::time_t cd = (std::time_t)card.due_date;
                         std::tm tmc;