Replace static hero image with animated rotating globe

Swap homepage hero from static isamples_planet.jpg to an animated
WebP showing the Cesium globe rotating with all 38K H3 cluster
data points (SESAR, OpenContext, GEOME, Smithsonian). 90 frames,
6-second seamless loop, 2.8 MB.

Addresses #55 (rotating globe request).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: rdhyee

assets/isamples_globe.webp

@@ -9,7 +9,7 @@ page-layout: full
 
 ::: {.column-page}
 
-[![Explore 6.7 million samples on an interactive globe](assets/isamples_planet.jpg){fig-alt="Globe showing iSamples data points from 4 scientific repositories" width="100%"}](/tutorials/progressive_globe.html "Explore the interactive globe")
+[![Explore 6.7 million samples on an interactive globe](assets/isamples_globe.webp){fig-alt="Animated rotating globe showing iSamples data points from 4 scientific repositories" width="100%"}](/tutorials/progressive_globe.html "Explore the interactive globe")
 
 :::