docs: README Overhaul + Screenshots

[ElBeh]

Description

The README was written for v0.8 and needs a full refresh to reflect the
current feature set (AlphaTab, GP Marker Import, Accordion Sidebar, Zoom,
Collapsible SyncOffsetEditor, etc.). Current screenshots are outdated and
don't show any notation features.

Requirements

• New screenshots covering all major UI states:
  • Practice Mode with notation (page + horizontal layout)
  • GP marker import dialog
  • Accordion sidebar (both open, one collapsed, sidebar collapsed)
  • SyncOffsetEditor (collapsed + expanded)
  • Zoom control in action
  • Band Sync viewer with notation cursor
• Rewrite feature descriptions to highlight notation/GP integration
• Update "What is SongLab?" intro paragraph
• Review Quick Start section for accuracy
• Consider adding a short GIF/video demo