perf: reduce main thread CPU usage and SSD wear from frequent JSON serialization

Sources/Firelink/AppSettings.swift

@@ -135,6 +135,7 @@ final class AppSettings: ObservableObject {
 
     private let defaults: UserDefaults
     private let storageKey = "Firelink.AppSettings.v1"
+    private var saveTask: Task<Void, Never>?
 
     init(defaults: UserDefaults = .standard) {
         self.defaults = defaults
@@ -247,9 +248,17 @@ final class AppSettings: ObservableObject {
             downloadDirectories: Dictionary(uniqueKeysWithValues: downloadDirectories.map { ($0.key.rawValue, $0.value) }),
             siteLogins: siteLogins
         )
+        let defaults = self.defaults
+        let storageKey = self.storageKey
 
-        if let data = try? JSONEncoder().encode(stored) {
-            defaults.set(data, forKey: storageKey)
+        saveTask?.cancel()
+        saveTask = Task { @MainActor [defaults, storageKey] in
+            let data = await Task.detached(priority: .background) {
+                try? JSONEncoder().encode(stored)
+            }.value
+            
+            guard !Task.isCancelled, let encoded = data else { return }
+            defaults.set(encoded, forKey: storageKey)
         }
     }
 

Sources/Firelink/DownloadController.swift

@@ -23,6 +23,7 @@ final class DownloadController: ObservableObject {
         let supportDir = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask).first ?? URL(fileURLWithPath: NSHomeDirectory())
         return supportDir.appendingPathComponent("Firelink").appendingPathComponent("downloads.json")
     }()
+    private var saveTask: Task<Void, Never>?
 
     init(settings: AppSettings) {
         self.settings = settings
@@ -674,14 +675,23 @@ final class DownloadController: ObservableObject {
     }
 
     private func saveDownloads() {
-        do {
-            let directory = storageURL.deletingLastPathComponent()
-            try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true, attributes: nil)
-            let state = StoredDownloadState(queues: queues, downloads: downloads)
-            let data = try JSONEncoder().encode(state)
-            try data.write(to: storageURL, options: .atomic)
-        } catch {
-            print("Failed to save downloads: \(error)")
+        let queuesCopy = queues
+        let downloadsCopy = downloads
+        let storageURL = self.storageURL
+
+        saveTask?.cancel()
+        saveTask = Task.detached(priority: .background) {
+            do {
+                let directory = storageURL.deletingLastPathComponent()
+                try FileManager.default.createDirectory(at: directory, withIntermediateDirectories: true, attributes: nil)
+                let state = StoredDownloadState(queues: queuesCopy, downloads: downloadsCopy)
+                let data = try JSONEncoder().encode(state)
+                
+                guard !Task.isCancelled else { return }
+                try data.write(to: storageURL, options: .atomic)
+            } catch {
+                print("Failed to save downloads: \(error)")
+            }
         }
     }
 

Sources/Firelink/DownloadTable.swift

@@ -64,6 +64,7 @@ final class TableSettings: ObservableObject {
 
     private let defaults = UserDefaults.standard
     private let storageKey = "Firelink.TableSettings.v1"
+    private var saveTask: Task<Void, Never>?
 
     init() {
         let defaultVisibleColumns: Set<DownloadColumn> = [.fileName, .size, .progress, .speed, .eta, .dateAdded]
@@ -90,8 +91,16 @@ final class TableSettings: ObservableObject {
             sortColumn: sortColumn,
             sortDirection: sortDirection
         )
-        if let data = try? JSONEncoder().encode(stored) {
-            defaults.set(data, forKey: storageKey)
+        let storageKey = self.storageKey
+        
+        saveTask?.cancel()
+        saveTask = Task { @MainActor in
+            let data = await Task.detached(priority: .background) {
+                try? JSONEncoder().encode(stored)
+            }.value
+            
+            guard !Task.isCancelled, let encoded = data else { return }
+            UserDefaults.standard.set(encoded, forKey: storageKey)
         }
     }
 }
@@ -114,7 +123,7 @@ struct DownloadTable: View {
 
     @StateObject private var tableSettings = TableSettings()
     @State private var pendingDeleteItems: Set<DownloadItem.ID>?
-    @State private var resizeBaseWidths: [DownloadColumn: CGFloat] = [:]
+    @State private var dragOffsets: [DownloadColumn: CGFloat] = [:]
     @State private var lastSelectedIndex: Int?
     @State private var draggedItemID: DownloadItem.ID?
 
@@ -283,12 +292,12 @@ struct DownloadTable: View {
                         .gesture(
                             DragGesture(minimumDistance: 1)
                                 .onChanged { value in
-                                    let baseWidth = resizeBaseWidths[column] ?? width(for: column)
-                                    resizeBaseWidths[column] = baseWidth
-                                    tableSettings.columnWidths[column] = max(70, baseWidth + value.translation.width)
+                                    dragOffsets[column] = value.translation.width
                                 }
-                                .onEnded { _ in
-                                    resizeBaseWidths[column] = nil
+                                .onEnded { value in
+                                    let baseWidth = tableSettings.columnWidths[column] ?? column.width
+                                    tableSettings.columnWidths[column] = max(70, baseWidth + value.translation.width)
+                                    dragOffsets[column] = nil
                                 }
                         )
                 }
@@ -445,7 +454,9 @@ struct DownloadTable: View {
     }
 
     private func width(for column: DownloadColumn) -> CGFloat {
-        max(70, tableSettings.columnWidths[column] ?? column.width)
+        let baseWidth = tableSettings.columnWidths[column] ?? column.width
+        let offset = dragOffsets[column] ?? 0
+        return max(70, baseWidth + offset)
     }
 
     private var sortedItems: [DownloadItem] {