fix: Prevent scrollbars in reader by hiding document overflow

Sets document.documentElement.style.overflow to 'hidden' when entering
the reader and restores it when leaving. This prevents scrollbars from
appearing during reader interactions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Gnathonic

src/lib/components/Reader/Reader.svelte

@@ -337,9 +337,14 @@
       });
     }
 
+    // Prevent scrollbars from appearing when in reader mode
+    document.documentElement.style.overflow = 'hidden';
+
     return () => {
       // Stop activity tracker when component unmounts
       activityTracker.stop();
+      // Restore overflow when leaving reader
+      document.documentElement.style.overflow = '';
     };
   });