Refresh README screenshots and document Dock recovery

[PathGao]

What changed

• Replaced both README screenshots with current Nifro examples, exported at the 1800px display width used by the README.
• Added matching English and Simplified Chinese warning callouts for the macOS state-transition issue that can make a fixed Dock auto-hide, including the killall Dock recovery command.

Why this approach

The screenshots remain JPEGs at 1800px wide because the README renders them at 900px. This keeps Retina clarity without adding the original 14 MB PNG captures to every clone.

Verification

• swift test — 222 tests in 49 suites passed.
• Confirmed both README variants reference the two replacement JPEGs, contain the WARNING callout and killall Dock, and contain no stale PNG image references.
• git diff --check passed.

Scope

Only the two README files and their two existing screenshot assets changed; no application behavior changed.