fix(dashboard,website): widen sidebar fallback to chat side panel; trim CSS comments

The chat page's mobile side panel reads the same --component-sidebar-background
skin var as App.tsx and renders transparent under skins that do not define it,
so it gets the same var(--background-base) fallback. Trim the two salvaged CSS
comments to the WHY.

web/src/pages/ChatPage.tsx

@@ -1764,7 +1764,7 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
             "border-l border-current/20 text-midground",
             "bg-background-base/95",
             "transition-transform duration-200 ease-out",
-            "[background:var(--component-sidebar-background)]",
+            "[background:var(--component-sidebar-background,var(--background-base))]",
             "[clip-path:var(--component-sidebar-clip-path)]",
             "[border-image:var(--component-sidebar-border-image)]",
             mobilePanelOpen