fix(wallet): wait for IPv6 port + retry start so a switch survives the DB-env race

Live logs showed the switch's new node starting on the correct wallet, reaching
"Verifying wallet…", then aborting with "Binding RPC on ::1 port 21769 failed" +
"Failed to rename wallet-savings.dat" — the OLD node's RPC port (on ::1/IPv6) and
Berkeley DB environment weren't fully released yet, so the wallet-verify DB
recovery couldn't rename the file. The app then reverted, and the connect loop
brought a node up on the DEFAULT wallet.

Two causes fixed:
- isPortInUse() only probed 127.0.0.1 (IPv4). The daemon also binds ::1 (IPv6),
  which lingers after IPv4 releases — so the readiness wait returned "free"
  prematurely. Now probe BOTH families (Windows: IPv4 + ::1 via in6addr_loopback;
  Linux: /proc/net/tcp + tcp6). mingw-verified.
- The datadir/DB-env can still be briefly held right after the old node exits, so
  the first start can abort. Retry the start (up to 6×, 2s backoff) with the
  CORRECT -wallet — active_wallet_file isn't reverted until we give up — resetting
  the crash count and re-arming -rescan each attempt, until one survives.

Also: the "Wallet switch failed" modal no longer repeats its title in the warning
header — it now shows the actual reason there.

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

src/app.cpp

@@ -3901,9 +3901,9 @@ void App::renderSwitchStopDaemonDialog()
             show_switch_stop_daemon_confirm_ = false;
         }
     } else if (failed) {
-        ui::material::DialogWarningHeader(TR("switch_progress_failed_title"));
-        ImGui::Dummy(ImVec2(0, ui::Layout::spacingSm()));
-        ImGui::TextWrapped("%s", wallet_switch_error_.c_str());
+        // The title already says "Wallet switch failed" — put the actual reason in the warning header
+        // rather than repeating the title.
+        ui::material::DialogWarningHeader(wallet_switch_error_.c_str());
         ImGui::Dummy(ImVec2(0, ui::Layout::spacingMd()));
         if (ui::material::TactileButton(TR("close"), ImVec2(110.0f * dp, 0))) {
             wallet_switch_dialog_open_.store(false);