690427:0812 Update Infras #01

.agents/skills/next-best-practices/i18n.md

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+# i18n (Thai / English)
+
+LCBP3 frontend **must not** hardcode Thai or English UI strings in components.
+
+## Rules
+
+1. **All user-facing strings go through the i18n layer** (`next-intl` / `i18next` — check `frontend/package.json`).
+2. **Keys use kebab-case**, namespaced by feature:
+   - `correspondence.list.title`
+   - `correspondence.form.submit`
+   - `common.actions.cancel`
+3. **Comments in code remain Thai** (business logic explanation); **only UI copy** goes through i18n.
+4. **Error messages** from backend (via ADR-007 `userMessage`) are already localized server-side — render them directly, don't translate client-side.
+
+---
+
+## ❌ Wrong
+
+```tsx
+export function CorrespondenceHeader() {
+  return <h1>รายการหนังสือติดต่อ</h1>; // ❌ hardcoded Thai
+}
+
+toast.success('บันทึกสำเร็จ'); // ❌ hardcoded
+```
+
+---
+
+## ✅ Right
+
+```tsx
+import { useTranslations } from 'next-intl';
+
+export function CorrespondenceHeader() {
+  const t = useTranslations('correspondence.list');
+  return <h1>{t('title')}</h1>;
+}
+
+toast.success(t('save.success'));
+```
+
+Translation files:
+
+```json
+// messages/th.json
+{
+  "correspondence": {
+    "list": { "title": "รายการหนังสือติดต่อ" },
+    "save":  { "success": "บันทึกสำเร็จ" }
+  }
+}
+
+// messages/en.json
+{
+  "correspondence": {
+    "list": { "title": "Correspondence List" },
+    "save":  { "success": "Saved successfully" }
+  }
+}
+```
+
+---
+
+## Zod Error Messages
+
+Zod error messages shown in forms **do** stay in Thai inline (per `specs/05-Engineering-Guidelines/05-03-frontend-guidelines.md`), because they're schema-bound and rarely need translation. If dual-language support becomes required, wrap with an i18n-aware resolver:
+
+```ts
+const schema = z.object({
+  projectUuid: z.string().uuid(t('validation.project.required')),
+});
+```
+
+---
+
+## Reference
+
+- [i18n Guidelines](../../../specs/05-Engineering-Guidelines/05-08-i18n-guidelines.md)
+- [Frontend Guidelines](../../../specs/05-Engineering-Guidelines/05-03-frontend-guidelines.md)