[upstream #16342] ClickHouse theme: sidebar and canvas surfaces

[berry-13]

Found in the ClickHouse theme audit for LibreChat-AI#16342. Rank 7 of 9 by visibility.

Click UI: Click UI's sidebar sits on sidebar.main.color.background.default #ffffff / #1f1f1c, with ~30px items (0.2813rem y padding), hover #e6e7e9 at 60% and active #e6e7e9 (ClickHouse/click-ui@537131f src/components/SidebarNavigationItem/SidebarNavigationItem.module.css, src/theme/styles/tokens-light.css:1125-1176, src/theme/styles/tokens-dark.css:1176-1185).

LibreChat under interface.theme: clickhouse: Under the theme the sidebar paints surface-primary-alt = background.split #f6f7fa / #282828, the canvas matches it, and conversation rows are 36px.

Gap class and why it is not in LibreChat-AI#16379: Theme data, but a maintainer call: surface-primary-alt is shared with other split surfaces, so moving it to white changes more than the sidebar. Row height is a component choice in stack-owned UnifiedSidebar/* and Conversations/*.

Screenshots (light, dark) are attached below.

[berry-13]

Waiting on its blocker before work starts: the mobile drawer's surface lines are in UnifiedSidebar.tsx, which open LibreChat-AI#16248 rewrites, and the list fade in ConversationsSection.tsx is shared by the drawer and the desktop panel, so moving only the desktop panel to a sidebar surface role would leave a mismatched fade in the drawer. Planned: a surface-sidebar role defaulting to surface-primary-alt, plus rows on the control height, once LibreChat-AI#16248 merges.