fix: add Session and Login

src/Session.tsx

@@ -0,0 +1,40 @@
+import { createContext, ReactNode, useContext, useLayoutEffect, useState } from "react";
+import { currentUser, User } from "./api";
+import { Login } from "./Login";
+
+export interface SessionProviderProps {
+  children: ReactNode;
+}
+
+const sessionContext = createContext<User | null>(null);
+
+export function SessionProvider(props: SessionProviderProps) {
+  const { children } = props;
+
+  const [user, setUser] = useState<User | null>(null);
+  const [err, setErr] = useState<unknown>(null);
+
+  function loadUser() {
+    currentUser()
+      .then((user) => { setUser(user); setErr(null); })
+      .catch((err) => { setUser(null); setErr(err); });
+  }
+
+  useLayoutEffect(() => { loadUser(); }, [err]);
+
+  function onLogin(user: User) {
+    setUser(user);
+    setErr(null);
+  }
+
+  let content: ReactNode;
+  if (!err && !user) content = <span className="loader" />;
+  else if (err) content = <Login onLogin={onLogin} />;
+  else content = <sessionContext.Provider value={user}>{children}</sessionContext.Provider>;
+
+  return content;
+}
+
+export function useSession() {
+  return useContext(sessionContext);
+}