Add /training page with four self-hosted walkthrough videos

- Videos loudness-normalized to -16 LUFS (originals were ~-33 LUFS).
- Static file serving rewritten to stream with HTTP Range support so
  video seeking works; .mp4/.webm MIME types added.
- Training nav links on homepage and /start, plus a first-time callout
  on /start; training page views tracked per source in admin analytics.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

public/training.js

@@ -0,0 +1,8 @@
+(async function(){
+  try{
+    const r=await fetch('/api/public/config');
+    if(!r.ok)return;
+    const c=await r.json();
+    if(c.siteName){document.title=`Training | ${c.siteName}`;const bn=document.getElementById('brandName');if(bn)bn.textContent=c.siteName}
+  }catch(e){}
+})();