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.
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