2.1.1

release: v2.1.1 - cursor hiding extends to wrapper, file action style

- input-handler.ts: when input is enabled, also hide the cursor on the
  video's parent (.video-wrapper). Setting cursor:none on the <video>
  alone left the OS cursor visible while the pointer was over the
  letterbox margins, which on `object-fit: contain` aspect ratios is a
  noticeable strip top/bottom or left/right. Restored to 'default' on
  disable so the toolbar etc. keep a normal pointer.
- session.css: file-action buttons styled as solid accent-cyan instead
  of translucent grey so "Mở file" / "Mở thư mục" stand out next to
  the file row's text and read as primary CTAs.
- README.md: document the synthetic-cursor architecture under a new
  "Tối ưu hóa Hiệu năng & Trải nghiệm" section.