fix: Scale floating button icons proportionally on mobile viewports

Remove hardcoded width/height HTML attributes from iconify-icon elements
that were overriding CSS sizing. The iconify-icon component uses HTML
attributes for SVG rendering, ignoring CSS width/height.

- Remove width="28" height="28" from 8 button templates
- Remove conflicting 768px media query from _buttons.css
- Add default desktop icon sizes (24px) in _scroll-behavior.css
- Icons now scale via clamp() from 18px (380px) to 24px (900px)

templates/partials/widgets/back-to-top.html

@@ -6,6 +6,6 @@
         data-tooltip="{{.UI.Widgets.BackToTop.Tooltip}}"
         style="display: none;"
         _="on click call scrollToTop(event)">
-    <iconify-icon icon="mdi:arrow-up" width="24" height="24"></iconify-icon>
+    <iconify-icon icon="mdi:arrow-up"></iconify-icon>
 </button>
 {{end}}