feat: add mobile compatibility

[grubmanItay]

Summary

• Hamburger menu replaces header buttons on mobile (<768px)
• Annotation panel renders as a full-screen overlay with backdrop and close button
• Touch support for resize handles, pinpoint annotations, text selection, and toolstrip
• Card action buttons are always visible on touch devices (no hover needed)
• Settings modal uses horizontal tab bar on mobile
• CommentPopover width capped to viewport
• Desktop layout (>=768px) completely unchanged

Files Changed

• New: useIsMobile hook, MobileMenu component
• Modified: App.tsx, AnnotationPanel, AnnotationToolstrip, CommentPopover, EditorAnnotationCard, ModeToggle, ResizeHandle, Settings, Viewer, usePinpoint, useResizablePanel

Test plan

• Desktop (>768px): verify no visual changes, all functionality works
• Mobile (<768px): Chrome DevTools device toolbar
  • Panel starts closed
  • Hamburger menu shows all options
  • Panel opens as overlay with backdrop
  • Toolstrip labels visible without hover
  • Card edit/delete buttons visible without hover
  • Text selection creates annotations
  • Pinpoint tap-to-annotate works
  • Settings modal renders with horizontal tabs
  • CommentPopover fits screen width

🤖 Generated with Claude Code

[backnotprop]

Ah this was on the todo list, great stuff