fix: make toggle functions tolerate null checkboxes for robustness

All three toggle functions (toggleCVLength, toggleIcons, toggleTheme) now
check if checkbox elements exist before setting their 'checked' property.

This prevents null pointer errors when:
- Menu checkboxes aren't rendered yet
- Functions are called programmatically before DOM is ready
- Tests call functions directly without full page render

Changed pattern from:
  set checkbox's checked to true

To:
  if checkbox exists
    set checkbox's checked to true
  end

Affected functions:
- toggleCVLength(): Lines 142-147, 154-159
- toggleIcons(): Lines 172-177, 183-188
- toggleTheme(): Lines 201-206, 212-217

The functions still work correctly when checkboxes exist, but now gracefully
handle missing elements instead of throwing null errors.

static/hyperscript/functions._hs

@@ -139,16 +139,24 @@ def toggleCVLength(isLong)
   if isLong is true
     remove .cv-short from paper
     add .cv-long to paper
-    set lengthCheckbox's checked to true
-    set menuLengthCheckbox's checked to true
+    if lengthCheckbox exists
+      set lengthCheckbox's checked to true
+    end
+    if menuLengthCheckbox exists
+      set menuLengthCheckbox's checked to true
+    end
     call localStorage.setItem('cv-length', 'long')
   end
 
   if isLong is false
     remove .cv-long from paper
     add .cv-short to paper
-    set lengthCheckbox's checked to false
-    set menuLengthCheckbox's checked to false
+    if lengthCheckbox exists
+      set lengthCheckbox's checked to false
+    end
+    if menuLengthCheckbox exists
+      set menuLengthCheckbox's checked to false
+    end
     call localStorage.setItem('cv-length', 'short')
   end
 end
@@ -161,15 +169,23 @@ def toggleIcons(showIcons)
   -- Update DOM state
   if showIcons is true
     add .show-icons to paper
-    set iconsCheckbox's checked to true
-    set menuIconsCheckbox's checked to true
+    if iconsCheckbox exists
+      set iconsCheckbox's checked to true
+    end
+    if menuIconsCheckbox exists
+      set menuIconsCheckbox's checked to true
+    end
     call localStorage.setItem('cv-icons', 'show')
   end
 
   if showIcons is false
     remove .show-icons from paper
-    set iconsCheckbox's checked to false
-    set menuIconsCheckbox's checked to false
+    if iconsCheckbox exists
+      set iconsCheckbox's checked to false
+    end
+    if menuIconsCheckbox exists
+      set menuIconsCheckbox's checked to false
+    end
     call localStorage.setItem('cv-icons', 'hide')
   end
 end
@@ -182,15 +198,23 @@ def toggleTheme(isClean)
   -- Update DOM state
   if isClean is true
     add .theme-clean to container
-    set themeCheckbox's checked to true
-    set menuThemeCheckbox's checked to true
+    if themeCheckbox exists
+      set themeCheckbox's checked to true
+    end
+    if menuThemeCheckbox exists
+      set menuThemeCheckbox's checked to true
+    end
     call localStorage.setItem('cv-theme', 'clean')
   end
 
   if isClean is false
     remove .theme-clean from container
-    set themeCheckbox's checked to false
-    set menuThemeCheckbox's checked to false
+    if themeCheckbox exists
+      set themeCheckbox's checked to false
+    end
+    if menuThemeCheckbox exists
+      set menuThemeCheckbox's checked to false
+    end
     call localStorage.setItem('cv-theme', 'default')
   end
 end