Add shared SettingsFormContainer/SettingsFormSection helpers that mirror
the Sources screen styling (uppercase subheadline headers, divider-
bracketed cards, ScrollView + LazyVStack) on macOS while keeping the
standard Form/Section layout on iOS and tvOS.
Convert PlaybackSettingsView and SubtitlesSettingsView to the new
helpers, wrap the macOS Settings detail pane in a NavigationStack so
NavigationLink pushes (Subtitles Appearance) render in the detail
column, fold the macOS-only Player Mode + Auto-resize player controls
into the Behavior section, and drop the unused queue footer.
Introduce PlatformMenuPicker that wraps short-option pickers in
LabeledContent + .pickerStyle(.menu) on tvOS so they render as a
compact dropdown instead of pushing a full-screen option list. On
iOS/macOS it falls through to a plain Picker, leaving rendering
unchanged.
Applied across Playback, Subtitles, Sidebar, Privacy, and Advanced
settings. Long language lists in PlaybackSettingsView are left as
push-style.
