Refactor classic sidebar to use sidebar.piml

[uyarburak]

sidebar.piml was introduced to dynamically generate the BrutalistSidebar, but the ClassicSidebar is still hardcoded. This risks inconsistencies if updates are made in the PIML and not mirrored in the classic sidebar.

Please refactor the ClassicSidebar to also use sidebar.piml as the shared source of truth.

[fezcode]

ClassicSidebar will be set as deprecated in the next major visual overhaul.
Will resolve this issue when that happens.