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.

README.md

@@ -102,6 +102,10 @@ To Be Played is split into three repositories:
 - **What's new**: a dialog on the dashboard shows the news entries the user has not seen yet, newest first. "Got it"
   marks them seen in Auth; closing the dialog any other way shows them again on the next dashboard visit. `/whats-new`
   (menu) lists all entries and marks them seen. Admins publish, preview, edit and delete entries on `/admin/news`.
+- **Welcome page**: `/welcome` opens once after the first login (Auth `settings.welcomeSeenAt` is null; only the
+  dashboard redirects, deep links stay). Feature cards with links, two next steps (Steam, import) with checks from
+  `/me` and the library summary. "Let's go" and "Skip" stamp it in Auth and open the dashboard. The settings page
+  links to it again.
 - **Correct game data** (AUDITOR or higher): "Correct data" on the game detail page changes name, header image,
   speedrun link, moods, endless flag and genres for every user; each field goes back to the source on its own.
   "Merge duplicate" joins two entries of the same game (e.g. the Steam game and the same game added by hand) for