Add drone silhouette background to all page heroes

Mirrors the index.html hero treatment across devkits, consultancy,
videos (dark strokes) and opensource (white strokes on dark bg).
.page-hero gets position:relative/overflow:hidden to contain them.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

style.css

@@ -300,6 +300,8 @@ img { display: block; max-width: 100%; }
 
 /* ── Page hero (inner pages) ─────────────────── */
 .page-hero {
+  position: relative;
+  overflow: hidden;
   padding: clamp(64px, 8vw, 100px) 0 clamp(56px, 7vw, 88px);
   border-bottom: 1px solid var(--rule);
 }