refactor: Extract modal backdrop close and scrollToTop to functions

- Add closeOnBackdrop(modal, evt) to utils._hs for modal backdrop clicks
- Add scrollToTop(evt) to utils._hs for smooth scroll to top
- Simplify 3 modal templates (shortcuts, info, pdf) from 4 lines to 1
- Simplify back-to-top button from 3 lines to 1

templates/partials/modals/shortcuts-modal.html

@@ -1,10 +1,7 @@
 {{define "shortcuts-modal"}}
 <!-- Keyboard Shortcuts Modal - Native Dialog -->
 <dialog id="shortcuts-modal" class="info-modal no-print"
-        _="on click
-             if event.target is me
-               call me.close()
-             end">
+        _="on click call closeOnBackdrop(me, event)">
     <div class="info-modal-content">
         <button class="info-modal-close" onclick="document.getElementById('shortcuts-modal').close()" aria-label="{{if eq .Lang "es"}}Cerrar{{else}}Close{{end}}">
             <iconify-icon icon="mdi:close" width="24" height="24"></iconify-icon>