Fix config data loss on startup and Codable forward-compatibility

- AppState.init: use _stocks = Published(wrappedValue:) to bypass didSet,
  preventing empty array from overwriting disk on decode failure
- AppState.saveStocks: guard against overwriting non-empty file with []
- Stock: explicit Codable extension using decodeIfPresent for optional fields,
  so missing fields in old JSON use defaults instead of failing the whole decode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

StockMonitor/Models/Stock.swift

@@ -12,7 +12,7 @@ enum Market: String, Codable, CaseIterable {
     }
 }
 
-struct Stock: Identifiable, Codable, Equatable {
+struct Stock: Identifiable, Equatable {
     var id: String              // 股票代码，如 "sh600000"、"usr_aapl"、"hk00700"
     var name: String
     var market: Market
@@ -31,3 +31,19 @@ struct Stock: Identifiable, Codable, Equatable {
         return quote.change * shares
     }
 }
+
+// MARK: - Codable（容错：未知字段用默认值，避免新版本解码老数据时整条失败）
+extension Stock: Codable {
+    enum CodingKeys: String, CodingKey {
+        case id, name, market, costPrice, holdingShares
+    }
+
+    init(from decoder: Decoder) throws {
+        let c = try decoder.container(keyedBy: CodingKeys.self)
+        id           = try c.decode(String.self,  forKey: .id)
+        name         = try c.decode(String.self,  forKey: .name)
+        market       = try c.decode(Market.self,  forKey: .market)
+        costPrice    = try c.decodeIfPresent(Double.self, forKey: .costPrice)
+        holdingShares = try c.decodeIfPresent(Double.self, forKey: .holdingShares)
+    }
+}

StockMonitor/State/AppState.swift

@@ -1,6 +1,7 @@
 import SwiftUI
 import Combine
 import AppKit
+import os
 
 @MainActor
 final class AppState: ObservableObject {
@@ -40,6 +41,8 @@ final class AppState: ObservableObject {
     }
 
     private func saveStocks(_ stocks: [Stock]) {
+        // 防止把空数组覆盖掉磁盘上有内容的文件
+        if stocks.isEmpty, !Self.loadStocks().isEmpty { return }
         guard let data = try? JSONEncoder().encode(stocks) else { return }
         Self.backupIfNeeded()
         try? data.write(to: Self.stocksFileURL, options: .atomic)
@@ -84,7 +87,8 @@ final class AppState: ObservableObject {
     init() {
         appLogger.info("AppState init start")
         logToFile("AppState init start")
-        self.stocks = Self.loadStocks()
+        // 用 _stocks 直接赋值，绕过 didSet，避免加载失败时把空数组覆盖写回磁盘
+        _stocks = Published(wrappedValue: Self.loadStocks())
         appLogger.info("AppState stocks loaded: \(self.stocks.count)")
         logToFile("AppState stocks loaded: \(self.stocks.count)")
         setupScheduler()

StockMonitor/StockMonitorApp.swift

@@ -1,6 +1,7 @@
 import SwiftUI
 import ServiceManagement
 import AppKit
+import os
 
 class AppDelegate: NSObject, NSApplicationDelegate {
     func applicationDidFinishLaunching(_ notification: Notification) {