fix(wallet): use localtime_s on Windows for the migration backup timestamp

beginAdoptSeedWallet used POSIX localtime_r for its thread-safe timestamp
(added in the Phase 2 review), which mingw/Windows lacks — it broke the
Windows cross-compile. Guard it: localtime_s on _WIN32, localtime_r elsewhere.
Latent since the Phase 2 commit (Linux-only builds didn't catch it).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app_network.cpp

@@ -3022,7 +3022,12 @@ void App::beginAdoptSeedWallet()
                 const std::string legacy = datadir + "/wallet.dat";
                 const std::string newWallet = base + "/DRAGONX/wallet.dat";
                 std::time_t t = std::time(nullptr);
-                std::tm tmv{}; localtime_r(&t, &tmv);  // thread-safe (the UI thread also uses localtime)
+                std::tm tmv{};  // thread-safe local time (the UI thread also uses localtime)
+#ifdef _WIN32
+                localtime_s(&tmv, &t);
+#else
+                localtime_r(&t, &tmv);
+#endif
                 char ts[32]; std::strftime(ts, sizeof(ts), "%Y%m%d-%H%M%S", &tmv);
                 const std::string backup = legacy + ".legacy-migrated-" + ts + ".bak";
                 if (!fs::exists(newWallet)) {