fix: Restore sticky action bar by using overflow-x: clip instead of hidden

Root cause: overflow-x: hidden on html/body elements breaks position: sticky
on descendant elements. This is a known CSS behavior.

Changes:
- _reset.css: Changed overflow-x from 'hidden' to 'clip' on html and body
  - 'clip' prevents horizontal scrolling WITHOUT breaking sticky positioning
- index.html: Restored hyperscript scroll handlers (initScrollBehavior, handleScroll)
- main.js: Disabled JavaScript scroll fallback in favor of hyperscript

Behavior:
- Desktop: Action bar hides on scroll down, reappears on scroll up
- Mobile (≤900px): Action bar stays visible at all times (CSS override)

Tested: Both desktop and mobile scroll behaviors work correctly

static/css/01-foundation/_reset.css

@@ -26,7 +26,7 @@ body {
   background-size: 40px 40px; /* For dark theme diagonal grid */
   background-attachment: fixed;
   max-width: 100vw; /* Prevent horizontal overflow */
-  overflow-x: hidden; /* NO horizontal scroll on mobile */
+  overflow-x: clip; /* Clip prevents horizontal scroll WITHOUT breaking position: sticky */
 }
 
 /* Smooth scrolling */
@@ -34,7 +34,7 @@ html {
   scroll-behavior: smooth;
   scroll-padding-top: 70px; /* Account for fixed header */
   max-width: 100vw; /* Prevent horizontal overflow */
-  overflow-x: hidden; /* NO horizontal scroll */
+  overflow-x: clip; /* Clip prevents horizontal scroll WITHOUT breaking position: sticky */
 }
 
 /* Ensure Iconify icons display properly */