fix(client): drop invalid li wrapper from sidebar groups

The group wrapper was rendering as `<li>`, but the enclosing `List` is
`component="nav"` and so renders a `<nav>`, not a `<ul>`. That put list items
directly inside a nav — invalid HTML, and a screen reader announces orphaned
list items with no enclosing list or item count.

`NavItem` renders a `div` via `ListItemButton`, so there is no list structure
for the group wrapper to join. Makes it a plain `Box` and drops the
now-redundant `listStyle: "none"`.

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

client/src/Components/sidebar/index.tsx

@@ -92,11 +92,7 @@ export const Sidebar = () => {
 						pb={theme.spacing(LAYOUT.MD)}
 					/>
 					{menu.map((group, groupIndex) => (
-						<Box
-							component="li"
-							key={group.group}
-							sx={{ listStyle: "none" }}
-						>
+						<Box key={group.group}>
 							{collapsed ? (
 								groupIndex > 0 && (
 									<Divider