Enforce theme at the window level instead of preferredColorScheme

SwiftUI's .preferredColorScheme on the root view could not revert from
dark back to light/system while a sheet was presented, and on macOS it
never applied to secondary window scenes such as Settings. The override
is now applied directly to UIKit/AppKit instead.

- SettingsManager.applyTheme sets overrideUserInterfaceStyle on every
  window of every connected scene (iOS) or NSApp.appearance (macOS);
  the theme setter calls it, so any future code path that changes the
  theme outside the setter must do the same
- AppTheme gains userInterfaceStyle / appearance mappings, with .system
  resolving to .unspecified / nil so the OS setting takes over again
- Applied on app appear for the initial launch state, and after both
  iCloud sync paths, since theme can arrive as a synced change and is
  no longer driven by SwiftUI state

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

Yattee/YatteeApp.swift

@@ -70,7 +70,6 @@ struct YatteeApp: App {
             ContentView()
                 .appEnvironment(appEnvironment)
                 #if !os(tvOS)
-                .preferredColorScheme(appEnvironment.settingsManager.theme.colorScheme)
                 .tint(appEnvironment.settingsManager.resolvedAccentColor)
                 #endif
                 #if os(macOS)
@@ -85,6 +84,9 @@ struct YatteeApp: App {
                     handleContinuedActivity(activity)
                 }
                 .onAppear {
+                    #if !os(tvOS)
+                    SettingsManager.applyTheme(appEnvironment.settingsManager.theme)
+                    #endif
                     registerBackgroundTasksIfNeeded()
                     #if os(tvOS)
                     TopShelfSnapshotWriter.startObserving(dataManager: appEnvironment.dataManager)