[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

OSGKeyboardShared/DesignSystem/ThemedRoot.swift

@@ -0,0 +1,51 @@
+// ThemedRoot.swift
+// OSGKeyboard · Design System
+//
+// `ThemedRoot` injects the right `ThemePalette` for the current system
+// colour scheme. Wrap the main App's root in this view to opt into
+// light/dark following; the keyboard extension deliberately stays dark
+// (Apple's custom keyboards always render dark) and does NOT use this.
+
+import SwiftUI
+
+public struct ThemedRoot<Content: View>: View {
+
+    @Environment(\.colorScheme) private var colorScheme
+
+    let content: () -> Content
+
+    public init(@ViewBuilder content: @escaping () -> Content) {
+        self.content = content
+    }
+
+    public var body: some View {
+        content()
+            .environment(\.themePalette, colorScheme == .dark ? Palette.dark : Palette.light)
+    }
+}
+
+#if DEBUG
+#Preview("ThemedRoot · Dark") {
+    ThemedRoot {
+        ZStack {
+            Palette.dark.background.ignoresSafeArea()
+            Text("Dark")
+                .foregroundStyle(Palette.dark.textPrimary)
+                .font(.title)
+        }
+    }
+    .preferredColorScheme(.dark)
+}
+
+#Preview("ThemedRoot · Light") {
+    ThemedRoot {
+        ZStack {
+            Palette.light.background.ignoresSafeArea()
+            Text("Light")
+                .foregroundStyle(Palette.light.textPrimary)
+                .font(.title)
+        }
+    }
+    .preferredColorScheme(.light)
+}
+#endif