feat(client): put the sidebar on paper

The sidebar set no background, so it inherited background.default. Once
that became an off-white ground rather than plain white, the nav read as
a grey slab against the content.

It now sits on background.paper, which also gives the layout a real
figure/ground: white nav, slightly darker content area behind it. The
existing right border and the mobile Backdrop still do their jobs, and
an opaque panel is the correct behaviour over that backdrop.

Claude-Session: https://claude.ai/code/session_01AbYLvwpEaZFXktHVcqrFRd

client/src/Components/sidebar/index.tsx

@@ -78,6 +78,10 @@ export const Sidebar = () => {
 					transition: transition,
 					zIndex: 1000,
 					backdropFilter: "blur(8px)",
+					// Sits on `paper` rather than inheriting the page ground, so
+					// the nav reads as a surface and the content area behind it
+					// stays the slightly darker `default`.
+					backgroundColor: theme.palette.background.paper,
 				}}
 			>
 				<List