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/pdf-modal.html

@@ -1,10 +1,7 @@
 {{define "pdf-modal"}}
 <!-- PDF Download Modal - Interactive Thumbnails -->
 <dialog id="pdf-modal" class="info-modal pdf-download-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" id="pdf-modal-content">
         <!-- Loading Overlay -->
         <div class="pdf-loading-overlay" id="pdf-loading-overlay">