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/shortcuts-button.html

@@ -6,6 +6,6 @@
     onclick="document.getElementById('shortcuts-modal').showModal()"
     aria-label="{{.UI.Widgets.Shortcuts.AriaLabel}}"
     data-tooltip="{{.UI.Widgets.Shortcuts.Tooltip}}">
-    <iconify-icon icon="mdi:keyboard-outline" width="28" height="28"></iconify-icon>
+    <iconify-icon icon="mdi:keyboard-outline"></iconify-icon>
 </button>
 {{end}}