refactor: reemplazar cpp-httplib por HTTP client minimalista

cpp-httplib requiere std::mutex que no compila con MinGW win32
thread model. Se reemplaza por http_client.cpp: sockets crudos,
sin threading, sin SSL, funciona en ambos thread models.
Elimina vendor/httplib.h (10K lineas). nlohmann/json se mantiene.

main.cpp: doble clic sin argumentos intenta la API en localhost:8484
automaticamente. Si falla muestra pantalla de error con boton Retry.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main.cpp

@@ -103,15 +103,8 @@ int main(int argc, char** argv) {
         fprintf(stderr, "Not found: %s\n", candidate.c_str());
     }
 
-    if (g_api_url.empty() && g_db_path.empty() && db_candidates.empty()) {
-        fprintf(stderr, "Usage: registry_dashboard [--api URL] [db_path ...]\n");
-        fprintf(stderr, "  --api URL   Connect to sqlite_api (default: http://127.0.0.1:8484)\n");
-        fprintf(stderr, "  db_path     Direct SQLite path(s) as fallback\n");
-        return 1;
-    }
-
-    if (g_db_path.empty() && !db_candidates.empty()) {
-        g_db_path = db_candidates.back();
+    if (!db_candidates.empty()) {
+        if (g_db_path.empty()) g_db_path = db_candidates.back();
     }
 
     reload_data();