docs: refresh README visuals for Kimi

[huajuan404]

Summary

• replace the README hero with a fresh live SwiftBar capture that includes Kimi
• regenerate the menu-opening GIF from the same source capture
• keep the demo generator pinned to the new reproducible source image

Test plan

• visually inspect hero.png
• inspect collapsed, transition, expanded, and closing GIF frames
• npm test
• npm run build
• doctor --dry-run, status --fixtures, menubar render
• npm pack --dry-run
• verify SwiftBar plugin remains executable