Fix exercise image slugs and add step-downs video

Rename exercise images to match nameToSlug() output (which strips
parentheticals) and add YouTube video for step-downs exercise.

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

Author: christianboer

…/calf-raises-slow-eccentric-off-step.png site/img/exercises/calf-raises.pngsite/img/exercises/calf-raises-slow-eccentric-off-step.png renamed to site/img/exercises/calf-raises.png site/img/exercises/calf-raises-slow-eccentric-off-step.png renamed to site/img/exercises/calf-raises.png

@@ -517,6 +517,7 @@ const EXERCISE_VIDEOS = {
     'dead-bug': 'eEhoSeBFoBk',
     'banded-clamshells': 'oHjBwnfpcQs',
     'single-leg-squat-to-bench': '19G5r-l1Q_I',
+    'step-downs': 'zeaVLglTeME',
 };
 
 function getVideoId(slug) {