Refine UI: add theme switcher, command palette hint, scrollbar styling, and responsive aside panel

[EmNudge]

Summary

• Add ThemeSwitcher to header using shared useTheme composable (removes duplicate state)
• Add ⌘K hint button for command palette discoverability
• Add close button to info aside panel
• Add custom scrollbar styling with theme-aware tokens
• Fix aside panel responsiveness on small screens (overlay instead of broken grid)
• Fix space key blocking page scroll when search input unfocused
• Replace <br> tags with proper CSS spacing