fix(keyboard): show standalone special keys with combo-only filter

[esphynox]

Summary

• keep standalone special keys visible when Show key combinations only is enabled and Special keys is on
• continue hiding unmodified regular keys under the combo-only filter
• add regression coverage for special-key and regular-key filtering behavior

Addresses #88 and #89

Tests

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

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

Standalone special keys like Escape and function keys remain visible when Show key combinations only is enabled and Special keys is on.