Fix Swift 6 warnings in usage stats and folder deletion

UsageStats/ModelUsageStat/ConversationUsageStat need explicit
nonisolated inits under this project's SWIFT_DEFAULT_ACTOR_ISOLATION =
MainActor setting, same as Message/Conversation. deleteFolder also
discarded FolderRecord.deleteOne's Bool result implicitly via the
write closure's return value.

oAI/Models/UsageStats.swift

@@ -24,13 +24,29 @@
 import Foundation
 
 struct UsageStats: Sendable {
-    var totalMessages: Int = 0
-    var totalTokens: Int = 0
-    var totalCost: Double = 0.0
-    var hasCostData: Bool = false
+    var totalMessages: Int
+    var totalTokens: Int
+    var totalCost: Double
+    var hasCostData: Bool
     var firstMessageDate: Date?
     var lastMessageDate: Date?
 
+    nonisolated init(
+        totalMessages: Int = 0,
+        totalTokens: Int = 0,
+        totalCost: Double = 0.0,
+        hasCostData: Bool = false,
+        firstMessageDate: Date? = nil,
+        lastMessageDate: Date? = nil
+    ) {
+        self.totalMessages = totalMessages
+        self.totalTokens = totalTokens
+        self.totalCost = totalCost
+        self.hasCostData = hasCostData
+        self.firstMessageDate = firstMessageDate
+        self.lastMessageDate = lastMessageDate
+    }
+
     var totalTokensDisplay: String {
         if totalTokens >= 1_000_000 {
             return String(format: "%.1fM", Double(totalTokens) / 1_000_000)
@@ -55,6 +71,22 @@ struct ModelUsageStat: Identifiable, Sendable {
     var hasCostData: Bool
     var lastUsed: Date
 
+    nonisolated init(
+        modelId: String,
+        messageCount: Int,
+        totalTokens: Int,
+        totalCost: Double,
+        hasCostData: Bool,
+        lastUsed: Date
+    ) {
+        self.modelId = modelId
+        self.messageCount = messageCount
+        self.totalTokens = totalTokens
+        self.totalCost = totalCost
+        self.hasCostData = hasCostData
+        self.lastUsed = lastUsed
+    }
+
     var totalTokensDisplay: String {
         if totalTokens >= 1_000_000 {
             return String(format: "%.1fM", Double(totalTokens) / 1_000_000)
@@ -79,6 +111,22 @@ struct ConversationUsageStat: Identifiable, Sendable {
     var totalCost: Double
     var hasCostData: Bool
 
+    nonisolated init(
+        conversationId: UUID,
+        name: String,
+        messageCount: Int,
+        totalTokens: Int,
+        totalCost: Double,
+        hasCostData: Bool
+    ) {
+        self.conversationId = conversationId
+        self.name = name
+        self.messageCount = messageCount
+        self.totalTokens = totalTokens
+        self.totalCost = totalCost
+        self.hasCostData = hasCostData
+    }
+
     var totalTokensDisplay: String {
         if totalTokens >= 1_000_000 {
             return String(format: "%.1fM", Double(totalTokens) / 1_000_000)

oAI/Services/DatabaseService.swift

@@ -639,7 +639,7 @@ final class DatabaseService: Sendable {
 
     nonisolated func deleteFolder(id: UUID) throws {
         try dbQueue.write { db in
-            try FolderRecord.deleteOne(db, key: id.uuidString)
+            _ = try FolderRecord.deleteOne(db, key: id.uuidString)
         }
     }