docs: add hero image to README

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -5,6 +5,13 @@
 
 An ESP32 firmware for the [M5Stack PaperColor](https://shop.m5stack.com/products/m5paper-color-esp32s3-dev-kit) e-ink display that turns it into a digital photo frame powered by your [Immich](https://immich.app/) server.
 
+<div align="center">
+  <figure>
+    <img src="docs/PaperColor.jpg" alt="PaperColor displaying a photo from Immich" width="400">
+    <figcaption><sub>PaperColor showing a dithered photo with location and date metadata overlays</sub></figcaption>
+  </figure>
+</div>
+
 ## Features
 
 - **Photo cycling modes** -- random, chronological, reverse-chronological, favorites-weighted, and recency-weighted