fix: 启动参数 duration 候选值对齐后端标准格式

前端 Select 使用 Go time.Duration.String() 标准字符串作为 value，
与后端查询/保存响应保持一致，解决保存后反显不匹配的问题。

frontend/src/__tests__/hooks/useSettings.test.tsx

@@ -31,7 +31,7 @@ const mockDesktopSettings: StartupSettings = {
       dbname: 'nex',
       maxIdleConns: 10,
       maxOpenConns: 100,
-      connMaxLifetime: '1h',
+      connMaxLifetime: '1h0m0s',
     },
     log: { level: 'info', path: '/home/user/.nex/log', maxSize: 100, maxBackups: 10, maxAge: 30, compress: true },
   },

frontend/src/__tests__/pages/Settings.test.tsx

@@ -41,7 +41,7 @@ const mockDesktopSettings: StartupSettings = {
       dbname: 'nex',
       maxIdleConns: 10,
       maxOpenConns: 100,
-      connMaxLifetime: '1h',
+      connMaxLifetime: '1h0m0s',
     },
     log: {
       level: 'info',

frontend/src/pages/Settings/StartupSettingsCard.tsx

@@ -9,18 +9,18 @@ const TIMEOUT_OPTIONS = [
   { label: '10 秒', value: '10s' },
   { label: '15 秒', value: '15s' },
   { label: '30 秒', value: '30s' },
-  { label: '1 分钟', value: '60s' },
-  { label: '2 分钟', value: '120s' },
-  { label: '5 分钟', value: '300s' },
+  { label: '1 分钟', value: '1m0s' },
+  { label: '2 分钟', value: '2m0s' },
+  { label: '5 分钟', value: '5m0s' },
 ]
 
 const CONN_LIFETIME_OPTIONS = [
-  { label: '5 分钟', value: '5m' },
-  { label: '15 分钟', value: '15m' },
-  { label: '30 分钟', value: '30m' },
-  { label: '1 小时', value: '1h' },
-  { label: '2 小时', value: '2h' },
-  { label: '4 小时', value: '4h' },
+  { label: '5 分钟', value: '5m0s' },
+  { label: '15 分钟', value: '15m0s' },
+  { label: '30 分钟', value: '30m0s' },
+  { label: '1 小时', value: '1h0m0s' },
+  { label: '2 小时', value: '2h0m0s' },
+  { label: '4 小时', value: '4h0m0s' },
 ]
 
 const MAX_AGE_OPTIONS = [