refactor: unify android error reporting (#5)

## Summary
Convert OpenIapError variants to object singletons with shared message
constants. Rename the purchase request type to RequestPurchaseParams and
update protocols, store flows, and tests to match.

Update the sample UI to surface purchase status banners with
PurchaseResultStatus and refresh cSpell dictionary entries.

Author: hyochan

.vscode/settings.json

@@ -39,5 +39,9 @@
     "typescript.validate.enable": false,
     "javascript.validate.enable": false,
     "typescript.tsc.autoDetect": "off",
-    "npm.autoDetect": "off"
-}
+    "npm.autoDetect": "off",
+    "cSpell.words": [
+        "billingclient",
+        "openiap"
+    ]
+}

Example/src/main/java/dev/hyo/martie/screens/AvailablePurchasesScreen.kt

@@ -7,6 +7,7 @@ import androidx.compose.foundation.lazy.LazyColumn
 import androidx.compose.foundation.lazy.items
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
 import androidx.compose.material.icons.filled.*
 import androidx.compose.material3.*
 import androidx.compose.runtime.*
@@ -22,6 +23,7 @@ import dev.hyo.martie.screens.uis.*
 import dev.hyo.openiap.IapContext
 import dev.hyo.openiap.store.OpenIapStore
 import dev.hyo.openiap.models.*
+import dev.hyo.openiap.store.PurchaseResultStatus
 import kotlinx.coroutines.launch
 
 @OptIn(ExperimentalMaterial3Api::class)
@@ -36,11 +38,7 @@ fun AvailablePurchasesScreen(
     val purchases by iapStore.availablePurchases.collectAsState()
     val status by iapStore.status.collectAsState()
     val connectionStatus by iapStore.connectionStatus.collectAsState()
-    
-    var showRestoreResult by remember { mutableStateOf(false) }
-    var restoreResultMessage by remember { mutableStateOf("") }
-    var showError by remember { mutableStateOf(false) }
-    var errorMessage by remember { mutableStateOf("") }
+    val statusMessage = status.lastPurchaseResult
 
     // Modal state
     var selectedPurchase by remember { mutableStateOf<OpenIapPurchase?>(null) }
@@ -67,7 +65,7 @@ fun AvailablePurchasesScreen(
                 title = { Text("Available Purchases") },
                 navigationIcon = {
                     IconButton(onClick = { navController.navigateUp() }) {
-                        Icon(Icons.Default.ArrowBack, contentDescription = "Back")
+                        Icon(Icons.AutoMirrored.Filled.ArrowBack, contentDescription = "Back")
                     }
                 },
                 actions = {
@@ -77,11 +75,15 @@ fun AvailablePurchasesScreen(
                             scope.launch {
                                 try {
                                     val restored = iapStore.restorePurchases()
-                                    showRestoreResult = true
-                                    restoreResultMessage = "Restored ${restored.size} purchases"
+                                    iapStore.postStatusMessage(
+                                        message = "Restored ${restored.size} purchases",
+                                        status = PurchaseResultStatus.SUCCESS
+                                    )
                                 } catch (e: Exception) {
-                                    showError = true
-                                    errorMessage = e.message ?: "Restore failed"
+                                    iapStore.postStatusMessage(
+                                        message = e.message ?: "Restore failed",
+                                        status = PurchaseResultStatus.ERROR
+                                    )
                                 }
                             }
                         },
@@ -161,12 +163,12 @@ fun AvailablePurchasesScreen(
                 }
             }
 
-            // Restore Result
-            if (showRestoreResult) {
-                item {
+            statusMessage?.let { result ->
+                item("status-message") {
                     PurchaseResultCard(
-                        message = restoreResultMessage,
-                        onDismiss = { showRestoreResult = false }
+                        message = result.message,
+                        status = result.status,
+                        onDismiss = { iapStore.clearStatusMessage() }
                     )
                 }
             }
@@ -214,16 +216,25 @@ fun AvailablePurchasesScreen(
                                 try {
                                     val ok = iapStore.finishTransaction(purchase, isConsumable)
                                     if (ok) {
-                                        showRestoreResult = true
-                                        restoreResultMessage = "Transaction finished successfully"
+                                        iapStore.postStatusMessage(
+                                            message = "Transaction finished successfully",
+                                            status = PurchaseResultStatus.SUCCESS,
+                                            productId = purchase.productId
+                                        )
                                         iapStore.getAvailablePurchases()
                                     } else {
-                                        showError = true
-                                        errorMessage = "Failed to finish transaction"
+                                        iapStore.postStatusMessage(
+                                            message = "Failed to finish transaction",
+                                            status = PurchaseResultStatus.ERROR,
+                                            productId = purchase.productId
+                                        )
                                     }
                                 } catch (e: Exception) {
-                                    showError = true
-                                    errorMessage = e.message ?: "Failed to finish transaction"
+                                    iapStore.postStatusMessage(
+                                        message = e.message ?: "Failed to finish transaction",
+                                        status = PurchaseResultStatus.ERROR,
+                                        productId = purchase.productId
+                                    )
                                 }
                             }
                         },
@@ -399,11 +410,15 @@ fun AvailablePurchasesScreen(
                             scope.launch {
                                 try {
                                     val restored = iapStore.restorePurchases()
-                                    showRestoreResult = true
-                                    restoreResultMessage = "Restored ${restored.size} purchases"
+                                    iapStore.postStatusMessage(
+                                        message = "Restored ${restored.size} purchases",
+                                        status = PurchaseResultStatus.SUCCESS
+                                    )
                                 } catch (e: Exception) {
-                                    showError = true
-                                    errorMessage = e.message ?: "Restore failed"
+                                    iapStore.postStatusMessage(
+                                        message = e.message ?: "Restore failed",
+                                        status = PurchaseResultStatus.ERROR
+                                    )
                                 }
                             }
                         },
@@ -419,20 +434,6 @@ fun AvailablePurchasesScreen(
         }
     }
 
-    // Error Dialog
-    if (showError) {
-        AlertDialog(
-            onDismissRequest = { showError = false },
-            title = { Text("Error") },
-            text = { Text(errorMessage) },
-            confirmButton = {
-                TextButton(onClick = { showError = false }) {
-                    Text("OK")
-                }
-            }
-        )
-    }
-    
     // Purchase Detail Modal
     selectedPurchase?.let { purchase ->
         PurchaseDetailModal(

Example/src/main/java/dev/hyo/martie/screens/OfferCodeScreen.kt

@@ -4,6 +4,7 @@ import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.*
 import androidx.compose.foundation.shape.RoundedCornerShape
 import androidx.compose.material.icons.Icons
+import androidx.compose.material.icons.automirrored.filled.ArrowBack
 import androidx.compose.material.icons.filled.*
 import androidx.compose.material3.*
 import androidx.compose.runtime.*
@@ -48,7 +49,7 @@ fun OfferCodeScreen(
                 title = { Text("Offer Code") },
                 navigationIcon = {
                     IconButton(onClick = { navController.navigateUp() }) {
-                        Icon(Icons.Default.ArrowBack, contentDescription = "Back")
+                        Icon(Icons.AutoMirrored.Filled.ArrowBack, contentDescription = "Back")
                     }
                 },
                 colors = TopAppBarDefaults.topAppBarColors(