Feature: Implement 'System Memory Usage Widget'

[Zalmez]

Description:
As a user, I want to be able to add a 'System Memory Usage Widget' to my dashboard.

Functionality:
Display the current RAM usage of the host system.

Reference:
This widget is part of the original Dashy feature set. More details can be found in the official documentation:
https://dashy.to/docs/widgets

Acceptance Criteria:

• A new widget option appears in the "Add Item" flow.
• The widget can be configured with the necessary options.
• The widget correctly displays the relevant information on the dashboard.