feat(welcome): add welcome page for new users after first login

Shows six feature cards and two next steps (Steam, import). The dashboard
redirects to it while Auth settings.welcomeSeenAt is null; "Let's go" and
"Skip" set the stamp. Settings links to it to reopen later.

src/utils/settings.js

@@ -51,3 +51,10 @@ export const saveSettings = (changes) => {
     queue = result.catch(() => {});
     return result;
 };
+
+/** PUT on Auth, 204: the welcome page was seen. The caller handles the response status. */
+export const markWelcomeSeen = () =>
+    apiFetch(`${import.meta.env.VITE_API_URL}/api/users/settings/welcome-seen`, {
+        method: 'PUT',
+        signal: AbortSignal.timeout(REQUEST_TIMEOUT_MS),
+    });