Extend opaque background to settings and other pushed pages on macOS

The NavigationStack-level background wasn't enough: macOS draws its own
translucent material behind pushed navigation pages, and Form/List
containers draw a translucent scroll background on top of anything
placed behind them, so several pages kept the wallpaper-tinted look.

Bake the opaque background into SettingsFormContainer (covers all pages
built on it, pushed or not) and add opaqueSettingsFormBackground(),
which hides the container's scroll background before applying the
opaque one, to every macOS-reachable Form/List settings page: player
controls and its sub-editors, sidebar settings, edit source, legacy
data import, subscription/playlist import, customize home and its
shortcut style page, and the log viewer. Contributors, translators and
the remote control device page get the plain opaque background.

Claude-Session: https://claude.ai/code/session_0154KH8RAVAvm6iVanhmoW8o

Yattee/Views/Settings/LegacyDataImportView.swift

@@ -46,6 +46,7 @@ struct LegacyAccountsImportView: View {
         #if os(iOS)
         .navigationBarTitleDisplayMode(.inline)
         #endif
+        .opaqueSettingsFormBackground()
         .toolbar {
             #if !os(tvOS)
             if showsDoneButton {