Pause polling and show a banner when the backend is unreachable

Adds a centralized BackendReachability singleton fed by a Dio
interceptor (connection-level failures only) and connectivity_plus.
PolledValue subscribes to it and pauses on offline / reloads
immediately on reconnect, replacing the previous per-card error
cascade. MainShell renders a global OfflineBanner with Retry and
Settings actions; cards downgrade error→stale when offline so the
banner is the only red element.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/windows/flutter/generated_plugin_registrant.cc

@@ -6,10 +6,13 @@
 
 #include "generated_plugin_registrant.h"
 
+#include <connectivity_plus/connectivity_plus_windows_plugin.h>
 #include <encrypter/encrypter_plugin_c_api.h>
 #include <url_launcher_windows/url_launcher_windows.h>
 
 void RegisterPlugins(flutter::PluginRegistry* registry) {
+  ConnectivityPlusWindowsPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("ConnectivityPlusWindowsPlugin"));
   EncrypterPluginCApiRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("EncrypterPluginCApi"));
   UrlLauncherWindowsRegisterWithRegistrar(