Add OG banner and mobile screenshots to README

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -1,6 +1,25 @@
-# fete
+<p align="center">
+  <img src="frontend/public/og-image.png" alt="fete" width="100%" />
+</p>
 
-A privacy-focused, self-hostable web app for event announcements and RSVPs. An alternative to Facebook Events or Telegram groups — reduced to the essentials.
+<p align="center">
+  <strong>Privacy-focused, self-hostable event announcements and RSVPs.</strong><br>
+  An alternative to Facebook Events or Telegram groups — reduced to the essentials.
+</p>
+
+<p align="center">
+  <img src="docs/screenshots/01-create-event.png" alt="Create Event" width="230" />
+  &nbsp;&nbsp;&nbsp;
+  <img src="docs/screenshots/02-event-detail.png" alt="Event Detail" width="230" />
+  &nbsp;&nbsp;&nbsp;
+  <img src="docs/screenshots/03-rsvp.png" alt="RSVP" width="230" />
+</p>
+
+<p align="center">
+  <sub>Create events &middot; Share with guests &middot; Collect RSVPs</sub>
+</p>
+
+---
 
 ## What it does