fix: use neutral gray background for dark mode

## Changes
- Replace MUI default blue-tinted dark background with neutral grays
  (#151518 for default, #1c1c21 for paper/cards)
- Remove decorative blue/purple SVG blob overlay from dark mode background

## Benefits
- Dark mode now has a clean, neutral gray appearance instead of a blue tint

client/src/Components/layout/AppLayout.tsx

@@ -47,10 +47,6 @@ const AppLayout = ({ children }: AppLayoutProps) => {
 			sx={{
 				minHeight: "100vh",
 				backgroundColor: theme.palette.background.default,
-				backgroundImage: mode === "dark" ? `url("${BackgroundSVG}")` : "none",
-				backgroundSize: "100% 100%",
-				backgroundPosition: "center",
-				backgroundRepeat: "no-repeat",
 			}}
 		>
 			<OfflineBanner visible={serverUnreachable} />

client/src/Utils/Theme/Palette.ts

@@ -31,4 +31,8 @@ export const darkPalette = {
 	secondary: {
 		main: colors.gray700,
 	},
+	background: {
+		default: "#151518",
+		paper: "#1c1c21",
+	},
 };