docs: refresh the README dashboard screenshot

The current dashboard (demo account) replaces the pre-v1.16 capture;
the JPEG also weighs a tenth of the old PNG.

README.md

@@ -22,7 +22,7 @@
   <a href="https://testflight.apple.com/join/bucuTBpa">iOS TestFlight</a>
 </p>
 
-<img width="2560" height="1600" alt="HealthLog dashboard: health metric tiles with 7-day trends and a weight chart, dark theme" src="docs/assets/dashboard-hero.png" />
+<img width="2434" height="1247" alt="HealthLog dashboard: health metric tiles with 7-day trends, blood-pressure and weight charts, dark theme" src="docs/assets/dashboard-hero.jpeg" />
 
 ## Why HealthLog