Removed the hide-scrollbar CSS class and all its usages throughout the
codebase. Hiding scrollbars can hurt UX by making it unclear when
content is scrollable.

Changes:
- Removed hide-scrollbar CSS definition from index.css
- Removed hide-scrollbar class from Navigation component (2 instances)
- Removed hide-scrollbar class from MemoDetailSidebar (2 instances)
- Removed hide-scrollbar class from TagsSection
- Removed hide-scrollbar class from ShortcutsSection

Components now use standard browser scrollbar behavior, which provides
better visual feedback to users about scrollable content. Modern
browsers already handle scrollbar appearance elegantly.