Improve initial active connection handling

If the last active connection is not reachable (server down, keychain problems,
token revoked, etc.), NeoChat currently fails to load at all, with the only fix
being to delete a line from the config file. This is surprisingly hard to fix with
a nice UX as long as we stick to the principle of loading the user's last active
connection automatically.

This patch thus drops that principle; instead, the user is always asked to choose
the connection to continue with.

src/login.cpp

@@ -100,7 +100,8 @@ void LoginHelper::init()
         Q_EMIT Controller::instance().globalErrorOccured(i18n("Network Error"), std::move(error));
     });
 
-    connectSingleShot(m_connection, &Connection::syncDone, this, []() {
+    connectSingleShot(m_connection, &Connection::syncDone, this, [this]() {
+        Q_EMIT loaded();
         Q_EMIT Controller::instance().initiated();
     });
 }