[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/OSGKeyboardApp.swift

@@ -1,7 +1,5 @@
 // OSGKeyboardApp.swift
 // OSGKeyboard · Main App
-//
-// DEBUG VERSION 2: restore real flow but instrument every step.
 
 import SwiftUI
 import OSGKeyboardShared
@@ -11,21 +9,22 @@ struct OSGKeyboardApp: App {
     @StateObject private var config = ProviderConfig.shared
 
     init() {
+        #if DEBUG
         print("🔥 [OSGKeyboardApp] init()")
+        #endif
     }
 
     var body: some Scene {
         WindowGroup {
-            Group {
-                if config.isConfigured {
-                    HomeView()
-                        .onAppear { print("🔥 [OSGKeyboardApp] → HomeView appeared") }
-                } else {
-                    OnboardingView(config: config)
-                        .onAppear { print("🔥 [OSGKeyboardApp] → OnboardingView appeared") }
+            ThemedRoot {
+                Group {
+                    if config.isConfigured {
+                        HomeView()
+                    } else {
+                        OnboardingView(config: config)
+                    }
                 }
             }
-            .onAppear { print("🔥 [OSGKeyboardApp] body appeared") }
         }
     }
-}
+}

OSGKeyboard/Views/HomeView.swift

@@ -33,7 +33,6 @@ struct HomeView: View {
         .sheet(isPresented: $showKeyboardPreview) {
             KeyboardPreviewSheet()
         }
-        .preferredColorScheme(.dark)
     }
 
     // MARK: - Header

OSGKeyboard/Views/KeyboardPreviewSheet.swift

@@ -42,7 +42,6 @@ struct KeyboardPreviewSheet: View {
                 keyboardBlock
             }
         }
-        .preferredColorScheme(.dark)
     }
 
     private var mockTextField: some View {

OSGKeyboard/Views/KeyboardPreviewStub.swift

@@ -30,7 +30,6 @@ struct KeyboardPreviewStub: View {
             .padding(.bottom, 6)
         }
         .frame(height: 280)
-        .preferredColorScheme(.dark)
     }
 
     // MARK: - Top bar

OSGKeyboard/Views/OnboardingView.swift

@@ -37,7 +37,6 @@ struct OnboardingView: View {
                     .padding(.bottom, Spacing.lg)
             }
         }
-        .preferredColorScheme(.dark)
     }
 
     private var pageDots: some View {

OSGKeyboard/Views/SettingsView.swift

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