feat(korea): add Market.krStock + KRW currency + chart x-axis

Adding Market.krStock cascades into several exhaustive switches that must all
update together: ExchangeRates.convert, DisplayCurrency, StockChartView ranges
and labels. Bundles tasks 2/3/4/5/14 into one build-passing landing.

- Market enum: add krStock case + kr_ prefix routing in Market.from
- ExchangeRates: add usdToKrw (fallback 1380), krwToCny derived, KRW path in convert
- DisplayCurrency: add .krw (₩), display name 韩元
- CurrencyService.fetchRates: include fx_susdkrw in Sina FX batch URL
- StockChartView: KR fullDayRange 0...389 (KST 09:00-15:30, 390 min); KR axis labels
  09:00/10:30/12:00/13:30/15:30

Tests: +5 Stock cases for KR, +8 CurrencyKRWTests covering all KR↔CNY/USD/HKD/KRW
conversion paths and fallback constant.

StockMonitor/Models/AppSettings.swift

@@ -4,12 +4,14 @@ enum DisplayCurrency: String, Codable, CaseIterable {
     case cny = "CNY"
     case hkd = "HKD"
     case usd = "USD"
+    case krw = "KRW"
 
     var symbol: String {
         switch self {
         case .cny: return "¥"
         case .hkd: return "HK$"
         case .usd: return "$"
+        case .krw: return "₩"
         }
     }
 
@@ -18,6 +20,7 @@ enum DisplayCurrency: String, Codable, CaseIterable {
         case .cny: return "人民币 ¥"
         case .hkd: return "港币 HK$"
         case .usd: return "美元 $"
+        case .krw: return "韩元 ₩"
         }
     }
 }

StockMonitor/Models/Stock.swift

@@ -4,10 +4,12 @@ enum Market: String, Codable, CaseIterable {
     case aStock  = "A股"
     case hkStock = "港股"
     case usStock = "美股"
+    case krStock = "韩股"
 
     static func from(code: String) -> Market {
         if code.hasPrefix("hk")   { return .hkStock }
         if code.hasPrefix("usr_") { return .usStock }
+        if code.hasPrefix("kr_")  { return .krStock }
         return .aStock
     }
 }

StockMonitor/Services/CurrencyService.swift

@@ -1,11 +1,14 @@
 import Foundation
 
 struct ExchangeRates {
-    var usdToCny: Double = 7.28   // 默认兜底值
+    var usdToCny: Double = 7.28      // 默认兜底值
     var usdToHkd: Double = 7.78
+    var usdToKrw: Double = 1380.0    // 默认兜底（2026 年初均值）；CurrencyService 拉成功后覆盖
 
     /// HKD/CNY = USD/CNY ÷ USD/HKD
     var hkdToCny: Double { usdToCny / usdToHkd }
+    /// KRW/CNY = USD/CNY ÷ USD/KRW
+    var krwToCny: Double { usdToCny / usdToKrw }
 
     /// 将 amount（native 货币）转换为目标货币
     func convert(_ amount: Double, from market: Market, to currency: DisplayCurrency) -> Double {
@@ -15,21 +18,23 @@ struct ExchangeRates {
         case .aStock:  inCny = amount
         case .hkStock: inCny = amount * hkdToCny
         case .usStock: inCny = amount * usdToCny
+        case .krStock: inCny = amount * krwToCny
         }
         // 再从 CNY 转出去
         switch currency {
         case .cny: return inCny
         case .hkd: return inCny / hkdToCny
         case .usd: return inCny / usdToCny
+        case .krw: return inCny / krwToCny
         }
     }
 }
 
 final class CurrencyService {
 
-    /// 从新浪财经获取 USD/CNY、USD/HKD 汇率，失败时返回内置兜底值
+    /// 从新浪财经获取 USD/CNY、USD/HKD、USD/KRW 汇率，失败时返回内置兜底值
     static func fetchRates() async -> ExchangeRates {
-        let urlStr = "https://hq.sinajs.cn/list=fx_susdcny,fx_susdhkd"
+        let urlStr = "https://hq.sinajs.cn/list=fx_susdcny,fx_susdhkd,fx_susdkrw"
         guard let url = URL(string: urlStr) else { return ExchangeRates() }
         var req = URLRequest(url: url)
         req.setValue("https://finance.sina.com.cn", forHTTPHeaderField: "Referer")
@@ -41,6 +46,7 @@ final class CurrencyService {
         for line in body.components(separatedBy: "\n") {
             if let r = parseFirstPositiveDouble(from: line, code: "fx_susdcny") { rates.usdToCny = r }
             if let r = parseFirstPositiveDouble(from: line, code: "fx_susdhkd") { rates.usdToHkd = r }
+            if let r = parseFirstPositiveDouble(from: line, code: "fx_susdkrw") { rates.usdToKrw = r }
         }
         return rates
     }

StockMonitor/Views/StockChartView.swift

@@ -22,6 +22,7 @@ struct StockChartView: View {
         case .aStock:  return 0...239
         case .hkStock: return 0...329
         case .usStock: return 0...959
+        case .krStock: return 0...389
         }
     }
 
@@ -35,6 +36,9 @@ struct StockChartView: View {
         case .usStock:
             // 基准 04:00 ET；330=09:30；720=16:00；959=19:59
             return [(0, "04:00"), (330, "09:30"), (570, "13:30"), (720, "16:00"), (959, "20:00")]
+        case .krStock:
+            // 基准 09:00 KST；180=12:00；389=15:29
+            return [(0, "09:00"), (90, "10:30"), (180, "12:00"), (270, "13:30"), (389, "15:30")]
         }
     }