refactor: use idiomatic hyperscript selector syntax

Replace verbose document.getElementById() and document.querySelectorAll()
with cleaner hyperscript syntax:
- #id for ID selectors
- .class and the first .class for class selectors
- <selector/> query literals for complex selectors
- #{variable} for dynamic ID interpolation

Files changed:
- utils._hs: scrollHeight, details, footer buttons, scrollToSection
- zoom._hs: all zoom control element selectors (14 changes)
- pdf-modal._hs: modal selector
- keyboard._hs: dynamic toggle and modal selectors
- contact-modal.html: response div and modal close
- index.html: ninja-keys bar selector

static/hyperscript/keyboard._hs

@@ -8,9 +8,9 @@
 -- Helper function to toggle a checkbox by ID (tries primary ID, then menu ID)
 -- Called from inline keyboard handler in body tag
 def handleToggleShortcut(toggleId, menuToggleId)
-  set toggle to document.getElementById(toggleId)
+  set toggle to #{toggleId}
   if toggle is null
-    set toggle to document.getElementById(menuToggleId)
+    set toggle to #{menuToggleId}
   end
   if toggle is not null
     set toggle.checked to (not toggle.checked)
@@ -23,7 +23,7 @@ end
 -- ==============================================================================
 -- Helper function to open a modal dialog by ID
 def openModalShortcut(modalId)
-  set modal to document.getElementById(modalId)
+  set modal to #{modalId}
   if modal is not null
     call modal.showModal()
   end