Enhancement: My Jetpack in Offline mode

### Impacted plugin

Jetpack

### What

Enable My Jetpack in Offline Mode with grayed out unavailable features

### How

Currently, when Jetpack is in Offline Mode, My Jetpack is removed from the menu. This can be a confusing experience in some situations. For example, when switching WP environment to `local` and attempting to reload My Jetpack will show the "Sorry, you are not allowed to access this page." message.
I propose re-enabling My Jetpack in the Offline Mode but "graying out" unavailable features as well as making sure the connection options are also blocked (impossible to connect or disconnect) like they are in the Jetpack dashboard.

The current state was introduced in: https://github.com/Automattic/jetpack/commit/3cf4ae640be8cf9cb53f37e8445bed7b62519a53

The issue could be related to the connection audit: https://github.com/Automattic/jetpack-roadmap/issues/1433

Start with the My Jetpack menu and its offline-mode behavior, then compare it with the Jetpack dashboard's connection controls. Review commit 3cf4ae640be8cf9cb53f37e8445bed7b62519a53 and the connection audit issue 1433. Done means My Jetpack remains accessible offline, unavailable features are grayed out, and connection actions are blocked.

Written by the indexing model from the issue text.