feat(nav): add header navigation and show Steam ID on dashboard

Header links to Dashboard, Library and Settings. The logo, the login page and
the landing page lead logged-in users to the dashboard. Expired tokens are
removed, and a 403 on the dashboard logs out. Remove the dev dummy token.

src/utils/auth.js

@@ -0,0 +1,26 @@
+const TOKEN_KEY = 'tbp_jwt';
+
+/**
+ * Only checks `exp`. The signature is checked by the backends.
+ */
+export function isTokenValid(token, nowMs = Date.now()) {
+    try {
+        const payload = token.split('.')[1].replace(/-/g, '+').replace(/_/g, '/');
+        const {exp} = JSON.parse(atob(payload));
+        return typeof exp === 'number' && exp * 1000 > nowMs;
+    } catch {
+        return false;
+    }
+}
+
+/**
+ * Returns the stored token, or null. Removes an expired token.
+ */
+export function getValidToken() {
+    const token = localStorage.getItem(TOKEN_KEY);
+    if (token && !isTokenValid(token)) {
+        localStorage.removeItem(TOKEN_KEY);
+        return null;
+    }
+    return token;
+}