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/css/main.css

@@ -4147,6 +4147,11 @@ html {
     color: #27ae60; /* Green icon on hover */
 }
 
+.print-friendly-btn.at-bottom {
+    opacity: 1;
+    background: #f39c12; /* Orange when at bottom */
+}
+
 /* Download Button (TOP POSITION) */
 .download-btn {
     position: fixed;
@@ -4190,6 +4195,11 @@ html {
     filter: brightness(0) invert(1); /* Keep white on hover */
 }
 
+.download-btn.at-bottom {
+    opacity: 1;
+    background: #f39c12; /* Orange when at bottom */
+}
+
 /* Mobile adjustments */
 @media (max-width: 768px) {
     .shortcuts-btn {