fix: restyle tab overflow chevrons to subtle ghost icons with fade masks

Replace prominent outlined chevron buttons with borderless ghost icons
and gradient fade masks for a cleaner overflow indicator. Reset shell-level
button custom properties to prevent global button styles from bleeding through.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/js/components/app-tabs.js

@@ -406,6 +406,8 @@ function initChevronScroll(nav) {
     const atEnd = nav.scrollLeft + nav.clientWidth >= nav.scrollWidth - SCROLL_THRESHOLD;
     btnStart.disabled = atStart;
     btnEnd.disabled = atEnd;
+    wrap.classList.toggle('at-start', atStart);
+    wrap.classList.toggle('at-end', atEnd);
   };
 
   bind(nav, 'scroll', update, { passive: true });