feat: add i18n support for Vietnamese and English

PROGRESS.md

@@ -137,6 +137,21 @@ Tham chiếu: `SOP_des.md` — Bản đặc tả hệ thống After-Sale CRM
 - [x] UserPolicy + RolePolicy (admin-only)
 - [x] Tests updated: 8 tests, 21 assertions pass
 
+### Internationalization (i18n) — Vietnamese + English
+- [x] Config: `APP_LOCALE=vi`, `APP_FALLBACK_LOCALE=en` in `.env`
+- [x] Created `lang/en/` and `lang/vi/` with 3 files each: `app.php`, `feedback.php`, `enums.php`
+- [x] ~120 translation keys covering: navigation, labels, status, widgets, services, notifications, enums
+- [x] All Resources: override `getPluralLabel()` + `getNavigationGroup()` with `__()` calls
+- [x] All Form fields: explicit `->label(__('key'))` on TextInput, Select, Textarea, etc.
+- [x] All Table columns: explicit `->label(__('key'))` on TextColumn, IconColumn
+- [x] All RelationManagers: `$title` property uses translation key
+- [x] All Widgets: `getHeading()`/`getDescription()` methods return translated strings
+- [x] Enums: `ContractType`, `ContractStatus`, `HandoverStatus`, `ServiceType` use `__()` in `label()`
+- [x] Services/Notifications: all user-facing strings use translation keys
+- [x] Blade views: all hardcoded strings replaced with `__()` calls
+- [x] Filament vendor translations (57 Vietnamese files) auto-activated for UI chrome
+- [x] Tests: 8/8 pass (21 assertions)
+
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 ## Database Schema (final — 23 migrations)