Add configurable copyright text with dynamic year to footer

Problem/Motivation

The Artisan starterkit's footer has no built-in copyright notice. Site builders currently have to hardcode copyright text directly in Twig templates, which means manually updating the year every year and duplicating effort across sub-themes.

There is no way to configure pre-text (e.g. "Copyright") or post-text (e.g. a site name or "All rights reserved") without editing template files directly.

Proposed resolution

Add two new theme settings to the Artisan theme settings form:

• copyright_pre_text — text displayed before the year (e.g. "© Copyright")
• copyright_post_text — text displayed after the year (e.g. "My Site. All rights reserved.")

The current year is resolved dynamically in the template so it never needs manual updating, the footer template then renders:

{{ copyright_pre }} [year] {{ copyright_post }}

Both fields are optional. If left empty, the block renders nothing (or only the year, depending on site builder preference).