feat(#34): added a text color change to the hover state

[svmukhin]

Closes #34

Added a text color change to the hover state to make the interaction clearly visible:

nav a:hover {
  background-color: var(--bg-tertiary);
  color: var(--button-bg-hover);
}

The --button-bg-hover token (#0860ca / #388bfd in dark mode) provides sufficient contrast and is already used for interactive element emphasis, making it a semantically appropriate choice here.

Also extended the transition property to include color:

transition: background-color 0.2s, color 0.2s;