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/color-theme-switcher.html

@@ -5,7 +5,7 @@
         class="color-theme-switcher no-print has-tooltip"
         aria-label="Toggle color theme"
         data-tooltip="Toggle color theme">
-  <iconify-icon id="themeIcon" icon="mdi:theme-light-dark" width="28" height="28"></iconify-icon>
+  <iconify-icon id="themeIcon" icon="mdi:theme-light-dark"></iconify-icon>
 </button>
 
 <!-- Hidden buttons for compatibility (not displayed) -->