feat: Display assigned user groups in Admin Panel

Description:
This PR adds the ability to view a user’s assigned groups in the Admin Panel when editing a user.

Backend Changes:
    Added a new endpoint:
    GET /api/v1/users/{user_id}/groups

        Returns the list of groups assigned to a specific user.
        Requires admin privileges.

Frontend Changes:
    Implemented getUserGroupsById API function to call the new backend endpoint, in lib/apis/users.

    Updated EditUserModal.svelte to:
        Load user groups asynchronously when the modal is opened.
        Display the groups inline in the form before the Save button.
        Show a loading state while fetching, and a “No groups assigned” message if none exist.

Result:
Admins can now see which groups a user belongs to directly from the edit user modal,
improving visibility and reducing the need to navigate away for group membership checks.

src/lib/apis/users/index.ts

@@ -443,3 +443,30 @@ export const updateUserById = async (token: string, userId: string, user: UserUp
 
 	return res;
 };
+
+export const getUserGroupsById = async (token: string, userId: string) => {
+	let error = null;
+
+	const res = await fetch(`${WEBUI_API_BASE_URL}/users/${userId}/groups`, {
+		method: 'GET',
+		headers: {
+			'Content-Type': 'application/json',
+			Authorization: `Bearer ${token}`
+		}
+	})
+		.then(async (res) => {
+			if (!res.ok) throw await res.json();
+			return res.json();
+		})
+		.catch((err) => {
+			console.error(err);
+			error = err.detail;
+			return null;
+		});
+
+	if (error) {
+		throw error;
+	}
+
+	return res;
+};