fix(keyboard): render fn as a single keycap

[esphynox]

Summary

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

Tests

• tuist generate
• xcodebuild test -project Keyty.xcodeproj -scheme Keyty -destination 'platform=macOS' — 301 tests, 0 failures
• Other: tuist generate not needed — no change to Project.swift, dependencies, build settings, resources, or project structure

UI Changes

No screenshots attached, but there are two visible changes:

• A single fn press now shows one keycap that lights on press and dims on release,
  instead of two separate keycaps.
• fn picks up the special-key theme instead of the modifier theme, following from it no
  longer sitting in the modifier row. Say the word if the modifier colors should be kept
  and the category mapping can be restored on its own.

Documentation

• Updated relevant docs
• No docs needed

Release Notes

Pressing fn now shows a single keycap instead of rendering it twice, and it is styled
like the other special keys.