fix: Apply at-bottom color to ALL buttons consistently

**Issue**: Only info and shortcuts buttons were showing orange color when
scrolled to bottom of page. Download PDF, Print Friendly, and Color Theme
buttons were missing this visual feedback.

**Fix**:
1. Added all buttons to at-bottom class logic in utils._hs:
   - #download-button
   - #print-friendly-button
   - .color-theme-switcher

2. Added CSS .at-bottom styles for missing buttons:
   - .download-btn.at-bottom
   - .print-friendly-btn.at-bottom
   - .color-theme-switcher.at-bottom

**Result**: All fixed buttons now show unified orange (#f39c12) background
when page is scrolled to bottom (within 50px), providing consistent visual
feedback across all controls.

Files changed:
- static/hyperscript/utils._hs: Added 3 buttons to scroll handler
- static/css/main.css: Added 2 .at-bottom styles
- static/css/color-theme.css: Added 1 .at-bottom style

static/hyperscript/utils._hs

@@ -114,12 +114,18 @@ def handleScroll()
     add .at-bottom to #back-to-top
     add .at-bottom to #info-button
     add .at-bottom to #shortcuts-button
+    add .at-bottom to #download-button
+    add .at-bottom to #print-friendly-button
+    add .at-bottom to .color-theme-switcher
   end
 
   if not isAtBottom
     remove .at-bottom from #back-to-top
     remove .at-bottom from #info-button
     remove .at-bottom from #shortcuts-button
+    remove .at-bottom from #download-button
+    remove .at-bottom from #print-friendly-button
+    remove .at-bottom from .color-theme-switcher
   end
 
   -- Update last scroll position