[P0-①] Theme follows system + keyboard background transparent

- Split Palette.dark / Palette.light into a ThemePalette struct
- Add EnvironmentKey<ThemePalette> + ThemedRoot<Content>
- Wrap OSGKeyboardApp root in ThemedRoot so main App follows system
- Remove 6 .preferredColorScheme(.dark) overrides from main App views
- KeyboardRootView drops its Palette.background fill, uses .background(Color.clear)
- Drop the unused 'background' computed property from KeyboardRootView
- Legacy 'Palette.xxx' static accessors still resolve to dark values,
  so 2000 lines of existing call sites stay untouched

xcodebuild iOS Simulator: SUCCEEDED

OSGKeyboard/Views/SettingsView.swift

@@ -37,7 +37,6 @@ struct SettingsView: View {
                         .foregroundStyle(Palette.accent)
                 }
             }
-            .preferredColorScheme(.dark)
         }
         .confirmationDialog(
             "Reset all settings?",