feat: polish first-launch onboarding and post-setup permission guidance

Streamline the five-step flow with smarter skip logic, a centered welcome intro, clearer zh copy, keyboard setup detection via the extension, and home tips when permissions are still missing after onboarding completes.

OSGKeyboardShared/Services/KeyboardSetupBridge.swift

@@ -0,0 +1,29 @@
+// KeyboardSetupBridge.swift
+// OSGKeyboard · Shared
+//
+// The main app cannot query iOS for installed keyboards. The extension
+// reports when it has appeared with Full Access so onboarding can skip
+// the manual setup step for returning users.
+
+import Foundation
+
+public enum KeyboardSetupBridge {
+    private enum Key {
+        static let fullAccessReady = "keyboard.extension.fullAccessReady"
+        static let lastSeenAt = "keyboard.extension.lastSeenAt"
+    }
+
+    /// True when the keyboard extension last appeared with Full Access enabled.
+    public static var isReadyForOnboardingSkip: Bool {
+        guard AppGroup.isAvailable else { return false }
+        return AppGroup.defaults.bool(forKey: Key.fullAccessReady)
+    }
+
+    /// Called from the keyboard extension on each appearance.
+    public static func markExtensionAppearance(hasFullAccess: Bool) {
+        guard AppGroup.isAvailable else { return }
+        let defaults = AppGroup.defaults
+        defaults.set(Date().timeIntervalSince1970, forKey: Key.lastSeenAt)
+        defaults.set(hasFullAccess, forKey: Key.fullAccessReady)
+    }
+}