Disable overscroll bounce effect on all pages

Add overscroll-behavior: none to html and body elements
to prevent rubber-band scrolling that shows extra space.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/styles/theme/index.css

@@ -21,6 +21,7 @@
 
 html {
   color-scheme: light dark;
+  overscroll-behavior: none;
 }
 
 body {
@@ -36,6 +37,7 @@ body {
   user-select: none;
   -webkit-user-select: none;
   overflow-x: hidden;
+  overscroll-behavior: none;
   accent-color: var(--color-accent);
 }