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

static/hyperscript/utils._hs

@@ -170,6 +170,25 @@ def setFooterHover(show)
   end
 end
 
+-- ==============================================================================
+-- MODAL HELPERS
+-- ==============================================================================
+-- Close modal when clicking backdrop (outside content)
+def closeOnBackdrop(modal, evt)
+  if evt.target is modal
+    call modal.close()
+  end
+end
+
+-- ==============================================================================
+-- SCROLL HELPERS
+-- ==============================================================================
+-- Smooth scroll to top of page
+def scrollToTop(evt)
+  call evt.preventDefault()
+  call window.scrollTo({top: 0, behavior: 'smooth'})
+end
+
 -- ==============================================================================
 -- NAVIGATION SCROLL
 -- ==============================================================================