feat: initialise a new totals tab with basic UI (#94)

* feat: initialise a new totals tab with basic UI

* fix: update group tabs and add stats page

* fix: styling within the new elements

* Prettier

* Display active user expenses only if active user is set

---------

Co-authored-by: Sebastien Castiel <sebastien@castiel.me>

src/lib/hooks.ts

@@ -48,3 +48,17 @@ export function useBaseUrl() {
   }, [])
   return baseUrl
 }
+
+/**
+ * @returns The active user, or `null` until it is fetched from local storage
+ */
+export function useActiveUser(groupId: string) {
+  const [activeUser, setActiveUser] = useState<string | null>(null)
+
+  useEffect(() => {
+    const activeUser = localStorage.getItem(`${groupId}-activeUser`)
+    if (activeUser) setActiveUser(activeUser)
+  }, [groupId])
+
+  return activeUser
+}