Inset/grouped list style doesn't work well with tvOS focus effects.
Always return .plain on tvOS and hide the list style picker from
appearance settings.
- Replace sheets with navigationDestination for Add/Edit Source on tvOS
(tvOS sheets have fixed size that doesn't fit the content)
- Fix focused cell clipping by replacing TVSettingsContainer's frame-based
layout with safeAreaInset, matching the main settings view pattern
- Use standard List with .listStyle(.grouped) for Sources on tvOS
- Add sidebar icons and titles to TVSettingsContainer for all settings
subviews, utilizing the left column space
- Remove redundant large navigation titles on tvOS (shown in sidebar)
- Move Edit Source Save button from toolbar into form above Delete button
for better tvOS focus navigation
These settings don't work well on Apple TV, so exclude the
ThemeSection, AccentColorSection, and the .preferredColorScheme/.tint
modifiers from tvOS builds.
