Add app lock, multi-account support, and redesigned account settings

App lock (local_auth) gates the app behind the OS's biometric/device
credential prompt via a new lock screen, layered on top of the existing
Nextcloud session rather than replacing it.

Multi-account support lets users save several Nextcloud logins and switch
between them (AccountStore, per-account browsing prefs, avatar swipe-to-
cycle); logout() now keeps an account's saved credentials/prefs intact so
it can be resumed from a "Continue as..." list, while removeAccount()
stays the only destructive path.

The account settings screen is redesigned around a single profile card
(avatar/name/email, server host, storage quota with groups) with
Refresh Cache/Logout/Remove actions, replacing the old separate "Server
Connection Info" section.

Also fixes activity-tab timestamps: the OCS Activity API reports time as
an ISO 8601 `datetime` string, not the numeric `timestamp` field the code
incorrectly assumed, which made every activity render as Jan 1 1970.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

windows/flutter/generated_plugin_registrant.cc

@@ -9,6 +9,7 @@
 #include <dynamic_color/dynamic_color_plugin_c_api.h>
 #include <file_saver/file_saver_plugin.h>
 #include <flutter_secure_storage_windows/flutter_secure_storage_windows_plugin.h>
+#include <local_auth_windows/local_auth_plugin.h>
 #include <pdfx/pdfx_plugin.h>
 #include <share_plus/share_plus_windows_plugin_c_api.h>
 #include <url_launcher_windows/url_launcher_windows.h>
@@ -20,6 +21,8 @@ void RegisterPlugins(flutter::PluginRegistry* registry) {
       registry->GetRegistrarForPlugin("FileSaverPlugin"));
   FlutterSecureStorageWindowsPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("FlutterSecureStorageWindowsPlugin"));
+  LocalAuthPluginRegisterWithRegistrar(
+      registry->GetRegistrarForPlugin("LocalAuthPlugin"));
   PdfxPluginRegisterWithRegistrar(
       registry->GetRegistrarForPlugin("PdfxPlugin"));
   SharePlusWindowsPluginCApiRegisterWithRegistrar(

windows/flutter/generated_plugins.cmake

@@ -6,6 +6,7 @@ list(APPEND FLUTTER_PLUGIN_LIST
   dynamic_color
   file_saver
   flutter_secure_storage_windows
+  local_auth_windows
   pdfx
   share_plus
   url_launcher_windows