fix(opencode): address issues found during OpenCode integration review

- Fix MCP server not removed from opencode.json when unchecked in edit modal
- Fix Windows atomic write failure when opencode.json already exists
- Fix i18n keys mismatch in OpenCodeFormFields (use opencode.* namespace)
- Fix unit test missing apps.opencode field assertion

src/i18n/locales/en.json

@@ -462,6 +462,7 @@
   },
   "opencode": {
     "npmPackage": "AI SDK Package",
+    "selectPackage": "Select a package",
     "npmPackageHint": "Select the npm package for communicating with the AI service",
     "baseUrl": "Base URL",
     "baseUrlHint": "Custom API endpoint URL",

src/i18n/locales/ja.json

@@ -462,6 +462,7 @@
   },
   "opencode": {
     "npmPackage": "AI SDK パッケージ",
+    "selectPackage": "パッケージを選択",
     "npmPackageHint": "AI サービスとの通信に使用する npm パッケージを選択",
     "baseUrl": "Base URL",
     "baseUrlHint": "カスタム API エンドポイント URL",

src/i18n/locales/zh.json

@@ -462,6 +462,7 @@
   },
   "opencode": {
     "npmPackage": "AI SDK 包",
+    "selectPackage": "选择一个包",
     "npmPackageHint": "选择用于与 AI 服务通信的 npm 包",
     "baseUrl": "Base URL",
     "baseUrlHint": "自定义 API 端点地址",