Refresh Nifro icon and project docs

[PathGao]

Summary

• adopt the new Icon Composer app icon and matching menu-bar template icon
• refresh the English and Simplified Chinese READMEs with clearer product language, practical use cases, and durable build instructions
• reorganize the English and Simplified Chinese roadmaps into Trying, Planned, Done, and Not Do, and replace one stale section reference in the site schema

Validation

• swift test — 220 tests in 44 suites passed
• Debug Xcode build completed with the new Nifro.icns generated from Nifro.icon
• git diff --check passed

Scope

This PR keeps the legacy AppIcon.appiconset and existing assets/icon*.png files as requested. It does not replace functional SF Symbols or website favicons.