feat: scaffolding dag_engine_ui (issue 0095 step 2)

ImGui app shell:
- main.cpp con fn::run_app(cfg, render), panel Main placeholder.
- CMakeLists.txt + fn_framework + fn_table_viz opt-in.
- app.md frontmatter base.

Capas HTTP/WS/tabs en commits siguientes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

main.cpp

@@ -0,0 +1,45 @@
+#include <imgui.h>
+#include "app_base.h"
+#include "core/panel_menu.h"
+#include "core/icons_tabler.h"
+#include "core/logger.h"
+// #include "viz/data_table.h"  // uncomment to enable data_table::render() panel
+
+// Toggles de paneles (visibles desde el menu View del menubar canonico)
+static bool g_show_main = true;
+
+static void draw_main() {
+    if (!ImGui::Begin(TI_HOME " Main", &g_show_main)) {
+        ImGui::End();
+        return;
+    }
+    ImGui::TextUnformatted("Hello from dag_engine_ui");
+    ImGui::End();
+}
+
+static void render() {
+    // El framework dibuja menubar (View/Layouts/Settings/About) y un
+    // DockSpaceOverViewport central (auto_dockspace=true por defecto).
+    // Aqui solo se dibujan los paneles propios de la app.
+    if (g_show_main) draw_main();
+
+    // === Data panel (uncomment to enable) ===
+    // static data_table::State data_state;
+    // static std::vector<data_table::TableInput> data_tables; // populate from your source
+    // data_table::render("main_data", data_tables, data_state);
+}
+
+int main(int /*argc*/, char** /*argv*/) {
+    static fn_ui::PanelToggle panels[] = {
+        { "Main", nullptr, &g_show_main },
+    };
+
+    fn::AppConfig cfg;
+    cfg.title       = "dag_engine_ui — Frontend ImGui para dag_engine. Lista, lanza e inspecciona DAGs con live updates via WS.";
+    cfg.about       = { "dag_engine_ui", "0.1.0", "Frontend ImGui para dag_engine. Lista, lanza e inspecciona DAGs con live updates via WS." };
+    cfg.log         = { "dag_engine_ui.log", 1 };
+    cfg.panels      = panels;
+    cfg.panel_count = sizeof(panels) / sizeof(panels[0]);
+
+    return fn::run_app(cfg, render);
+}