Fix cloud features staying off after login when the setup wizard was closed

Reported on #15239: after signing in to Orca Cloud on a fresh install, no sync
prompt appears and File > Sync Presets is greyed out with nothing to explain why.

CAUSE. AppConfig::get_stealth_mode() returns true whenever `firstguide/finish` is
unset, and that flag is written in exactly one place — GuideFrame::SaveProfile(),
i.e. only when the setup wizard is COMPLETED. Closing the wizard is what people do
today to reach the login (the wizard never offers it, which is #15239 itself), so a
new user ends up permanently in a stealth mode they never chose. Every cloud gate
keyed on get_stealth_mode() then switches off silently, including:

  * GUI_App::on_user_login_handle(), which returns EARLY on stealth — so the whole
    post-login flow is skipped: preset migration, plugin fetch, user-preset load and
    show_sync_dialog(). That is the missing sync prompt.
  * the Sync Presets item in both the top menu and the File menu, whose enable
    lambda was `is_user_login() && !get_stealth_mode()`. That is the greyed item.

The result is indistinguishable from real Stealth mode, and nothing in the UI says
so, because the one place that DOES explain it — the "Quit Stealth Mode" dialog in
handle_web_request() — only covers the homepage login commands.

FIX, two parts.

1. The pre-wizard value is a DEFAULT for "the user has not been asked yet", not a
   setting, so it must not survive the user answering. Signing in to a cloud account
   is that answer. AppConfig now carries a session-only `m_cloud_logged_in` mirrored
   from the network agent (on login, on logout, and at agent start so a restored
   session counts), and get_stealth_mode() consults it before falling back to the
   pre-wizard default. An explicit Stealth mode setting is untouched and still wins:
   a user who turned it on deliberately stays offline whether or not they sign in.

2. Sync Presets no longer greys itself out. Both refusal paths already had a message
   to show — "You must be logged in…" and now one for Stealth mode naming the
   Preferences toggle — and the enable lambda was making both unreachable. A disabled
   item that cannot say why is the reason this took a bug report to find.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/libslic3r/AppConfig.hpp

@@ -91,6 +91,9 @@ public:
 	std::string get_language_code();
 	std::string get_hms_host();
 	bool get_stealth_mode();
+	// Session state, not a setting: mirrors whether a cloud account is currently signed in, so
+	// get_stealth_mode() can tell "the user has not been asked yet" from "the user said no".
+	void set_cloud_logged_in(bool logged_in) { m_cloud_logged_in = logged_in; }
 	bool get_hide_login_side_panel();
 
 	// Clear and reset to defaults.
@@ -424,6 +427,9 @@ private:
 	Semver                                                      m_orig_version;
 	// Whether the existing version is before system profiles & configuration updating
 	bool                                                        m_legacy_datadir;
+	// A cloud account is signed in right now. Deliberately NOT persisted: it is mirrored from the
+	// network agent on login/logout, so a stale value cannot outlive the session that set it.
+	bool                                                        m_cloud_logged_in{false};
 
 	std::string                                                 m_loading_path;