Modernize the settings window UI

[hisaac]

The settings window works but was, in the author's words, slapped together into something only
somewhat presentable. Bring it up to current macOS conventions.

What is off today

• Fixed .frame(width: 480, height: 300) with .padding([.leading, .trailing], 40) bolted on.
  Hard-coded metrics like this break under longer localised strings and larger accessibility text
  sizes, and they are why the padding needed hand-tuning in the first place.
• A hand-rolled settingDescription() applying .font(.system(size: 11.0)) and
  .foregroundStyle(.secondary). Fixed point sizes do not scale; there are semantic styles for this.
• LabeledContent used as a layout device, with labels like "Behavior:" attached to unrelated
  controls, which is why the alignment is uneven.
• About and Quit buttons inside the settings window. Unusual on macOS, where both live in the app
  menu. Note the nuance though — see below.
• The window is not resizable and does not size itself to its content.

Worth revisiting

Closed issue #4 covers the About/Quit buttons and is directly relevant: they exist because in
headless mode there is no app menu to reach them from. The suggestion there — following Moom, and
showing them only in headless mode — is a better answer than either always or never showing them, and
this is the natural time to do it.

Direction

Form with .formStyle(.grouped) gets most of the way to a standard macOS settings pane. Prefer
semantic fonts and standard spacing over fixed values, and let the window size itself.

If the xib removal lands first, this becomes a Settings scene, which also brings the standard
window behaviour and title for free.

Verify

• Looks correct at default text size and at larger accessibility sizes.
• No clipping with longer strings.
• Light and dark appearance both fine.
• Keyboard navigation reaches every control in a sensible order.
• Still usable in headless mode, where the app menu is unavailable.