perf: enable lite-yt-embed, optimize avatar, add cache headers

- YouTube embeds now use lite-youtube facade (loads iframe on click,
  ~800 KiB savings per page with embedded videos)
- Avatar resized from 1000x1000 to 400x400 (152 KiB → 39 KiB)
- lite-yt-embed.css max-width changed to 100% for responsive layout
- Removed unused Tailwind primary color palette from CSS bundle

Confab-Link: http://localhost:8080/sessions/edb1b7b0-da66-4486-bd9c-d1cfa7553b88

css/lite-yt-embed.css

@@ -6,7 +6,7 @@ lite-youtube {
     background-position: center center;
     background-size: cover;
     cursor: pointer;
-    max-width: 720px;
+    max-width: 100%;
 }
 
 /* gradient */