@layout-sider-background cannot set to linear-gradient

Editor assessment

The subject states the defect precisely — the `@layout-sider-background` Less variable cannot be set to a linear-gradient — naming the exact token and the limitation, so the change is understandable without a diff. The footer `close #26755` is well formed and links the work to its reporting issue, which documents the desired use case the absent body would carry. Type `fix` is accurate; absent scope is tolerable since the variable identifies the Layout area. No body prose exists, acceptable given the issue link. Style is clean — no trailing period, no junk. A clear, traceable, publishable small fix.