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).

data.h

@@ -0,0 +1,86 @@
+#pragma once
+
+#include <mutex>
+#include <set>
+#include <string>
+#include <vector>
+
+namespace kanban {
+
+struct Issue {
+    std::string id;
+    std::string title;
+    std::string status;
+    std::string type;
+    std::string scope;
+    std::string priority;
+    std::vector<std::string> domain;
+    std::vector<std::string> tags;
+    std::vector<std::string> depends;
+    std::vector<std::string> blocks;
+    std::vector<std::string> related;
+    std::string flow;
+    std::string file_path;
+    bool        completed = false;
+    std::string body;  // only filled in detail GET
+};
+
+struct Flow {
+    std::string id;
+    std::string title;
+    std::string status;
+    std::string kind;
+    std::vector<std::string> tags;
+    std::string file_path;
+    std::string body;
+};
+
+struct Meta {
+    std::vector<std::string> statuses;
+    std::vector<std::string> priorities;
+    std::vector<std::string> scopes;
+    std::vector<std::string> types;
+};
+
+struct Filters {
+    std::set<std::string> domains;
+    std::set<std::string> scopes;
+    std::set<std::string> priorities;
+    std::set<std::string> tags;
+    bool include_completed = false;
+};
+
+struct State {
+    std::mutex                 mu;
+    std::string                backend_url    = "http://127.0.0.1:8487";
+    std::vector<Issue>         issues;
+    std::vector<Flow>          flows;
+    Meta                       meta;
+    Filters                    filters;
+    std::string                selected_issue_id;
+    Issue                      selected_issue_detail;
+    bool                       loading        = false;
+    std::string                last_error;
+    long long                  last_refresh_ns = 0;
+};
+
+State& state();
+
+// HTTP operations (block briefly, called from background or coalesced).
+bool refresh_issues();
+bool refresh_flows();
+bool refresh_meta();
+bool refresh_issue_detail(const std::string& id);
+bool patch_issue_status(const std::string& id, const std::string& new_status);
+bool patch_issue_fields(const std::string& id, const std::string& json_partial);
+
+// Background SSE subscription (no-op if already started).
+void start_sse();
+void stop_sse();
+
+// Filters helpers.
+bool passes_filters(const Issue& iss);
+std::vector<std::string> collect_domains();
+std::vector<std::string> collect_tags();
+
+}  // namespace kanban