feat: initial scaffold of kanban_cpp v2 (issue 0130)

Frontend C++ ImGui (main.cpp + 4 paneles) + backend Go (HTTP + SQLite + fsnotify + SSE).
Reusa parse/scan/watch funcs del registry (issue 0130a).

panels.h

@@ -0,0 +1,10 @@
+#pragma once
+
+namespace kanban {
+
+void draw_board();
+void draw_flows();
+void draw_filters();
+void draw_detail();
+
+}  // namespace kanban