feat(T01): auth + user management + RBAC + base frontend + i18n

- Backend: FastAPI, JWT auth (HS256), bcrypt, RBAC middleware
- User CRUD (admin-only), soft-delete, pagination
- Pydantic BaseSettings config, async SQLAlchemy 2.0
- 50 backend tests, 88% coverage
- Frontend: Next.js 14 App Router, Tailwind, design tokens
- Login page, auth context, API client with auto-refresh
- i18n: next-intl, DE/EN (31 keys each)
- 6 base UI components (Button, Input, Card, Table, Modal, Toast)
- 12 frontend tests, npm build success

frontend/messages/en.json

@@ -0,0 +1,33 @@
+{
+  "auth.loginTitle": "Sign In",
+  "auth.email": "Email Address",
+  "auth.password": "Password",
+  "auth.loginButton": "Sign In",
+  "auth.logout": "Sign Out",
+  "auth.loginSuccess": "Successfully signed in",
+  "auth.error.loginFailed": "Login failed. Please check your credentials.",
+  "auth.error.emailRequired": "Email address is required",
+  "auth.error.emailInvalid": "Please enter a valid email address",
+  "auth.error.passwordRequired": "Password is required",
+  "auth.welcome": "Welcome back",
+  "common.save": "Save",
+  "common.cancel": "Cancel",
+  "common.delete": "Delete",
+  "common.edit": "Edit",
+  "common.confirm": "Confirm",
+  "common.search": "Search",
+  "common.loading": "Loading...",
+  "common.noData": "No data available",
+  "nav.vehicles": "Vehicles",
+  "nav.contacts": "Contacts",
+  "nav.sales": "Sales",
+  "nav.settings": "Settings",
+  "nav.dashboard": "Dashboard",
+  "nav.ocr": "OCR Scan",
+  "nav.copilot": "AI Assistant",
+  "user.role.admin": "Administrator",
+  "user.role.verkaeufer": "Sales",
+  "user.role.buchhaltung": "Accounting",
+  "user.active": "Active",
+  "user.inactive": "Inactive"
+}