fix: rework mobile nav as fixed overlay, add 3-line burger

- Nav menu is now a fixed fullscreen overlay (opacity fade)
- Header stays at 3.5rem height, no height animation
- Burger has 3 lines, middle fades out on open
- Body scroll locked with overflow: hidden when menu open
- Various mobile layout fixes (handles, system, footer padding)
- Header full-width with no border-radius on mobile

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

assets/css/src/_header.scss

@@ -1,10 +1,11 @@
 body {
   > header {
+    box-sizing: border-box;
     position: sticky;
     z-index: 999;
     top: 0;
     left: 0;
-    width: 100%;
+    width: 100vw;
     padding: 1.5rem var(--padding-body);
     box-sizing: border-box;