fix(keyboard): normalize fn key rendering

[esphynox]

Summary

Fix duplicate and inconsistent fn rendering by moving it to a single state-driven path.

• keep classical modifier handling limited to command/shift/option/control
• render physical fn from flagsChanged state updates instead of treating it like a normal chord modifier
• suppress .function keystroke rendering and retain the active fn group through release so it does not duplicate
• add focused keyboard visualizer coverage for fn press/release and transformed-key behavior

Tests

• tuist generate
• xcodebuild test -project Keyty.xcodeproj -scheme Keyty -destination 'platform=macOS'
• Other:
  • xcodebuild test -project Keyty.xcodeproj -scheme Keyty -destination 'platform=macOS' -only-testing:KeytyTests/KeyboardVisualizerTests -only-testing:KeytyTests/KeycapItemFactoryTests

Run project commands from Apps/Keyty.

UI Changes

Attach screenshots or screen recordings for visible UI changes. Write N/A if there are none.

Before	After
N/A	N/A

Documentation

• Updated relevant docs
• No docs needed

Release Notes

Fix duplicated and inconsistent fn key rendering in the keyboard visualizer.