This update replaces the old dirty_contacts_expiry timer with a smarter, debounced system for saving contacts.

• A new method, markContactsChanged(), keeps track of when contact data is modified.
• Contacts are written to storage after one minute of no changes, or at least every five minutes.
• The onContactDataChanged() hook is now called across update paths in BaseChatMesh and MyMesh to automatically flag changes when important fields are updated.
• The shutdown and reboot logic in the UI now forces a final flush before powering off.

Overall, this makes contact persistence more reliable while keeping unnecessary writes to a minimum.