Fix SubscriptionCallback interface implementation in PremiumFeatureManager

- Replace lambda callback with proper SubscriptionCallback interface implementation
- Implement all three required callback methods:
  * onSuccess(message: String) - invokes onPurchase callback on successful purchase
  * onError(error: String) - handles purchase errors silently
  * onUserCancelled() - handles user cancellation

- This matches the RevenueCatManager.purchasePremium() function signature
- Resolves "Argument type mismatch" compilation error

All PremiumFeatureManager compilation errors are now resolved

Author: claude

app/src/main/java/com/kharagedition/tibetankeyboard/ai/PremiumFeatureManager.kt

@@ -103,11 +103,19 @@ class PremiumFeatureManager(private val context: Context) {
             .setMessage(message)
             .setIcon(android.R.drawable.ic_dialog_info)
             .setPositiveButton("Upgrade") { _, _ ->
-                revenueCatManager.purchasePremium(activity) { success ->
-                    if (success) {
+                revenueCatManager.purchasePremium(activity, object : com.kharagedition.tibetankeyboard.subscription.RevenueCatManager.SubscriptionCallback {
+                    override fun onSuccess(message: String) {
                         onPurchase?.invoke()
                     }
-                }
+
+                    override fun onError(error: String) {
+                        // Handle error silently
+                    }
+
+                    override fun onUserCancelled() {
+                        // User cancelled the purchase
+                    }
+                })
             }
             .setNegativeButton("Cancel") { dialog, _ ->
                 dialog.dismiss()