feat(unit-2-auth): Implement authentication pages and guards

Completes Unit 2 Code Generation (Steps 1-21 of 24):
- SetupPage: First Owner account creation with language preference
- InviteCompletePage: User invitation completion with token validation
- InitGuard: System initialization status checking
- RoleGuard: Role-based access control for protected routes
- API hooks: useSetup, useValidateInvitation, useCompleteInvitation
- Shared password validation schema via Zod
- Error display components: FormErrorBanner, FieldError
- MSW mock handlers for setup and invitation flows
- i18n translations (en/nl) for auth screens
- Router updates: public routes (/setup, /invite/complete) + role guards
- Unit/integration tests: 25/29 passing (86% pass rate)
- Build: ✅ PASSED, Lint: ✅ PASSED, Tests: 86% PASSED

Stories: US-01, US-02, US-03, US-04, US-05, US-06, US-07, US-13, US-14
Remaining: Step 22 (README), Step 23 (final verification), Step 24 (commit)

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/i18n/locales/en/translation.json

@@ -31,46 +31,25 @@
     },
     "setup": {
         "title": "System Setup",
-        "subtitle": "Create the first Owner account to get started.",
-        "fields": {
-            "name": "Full name",
-            "email": "Email",
-            "password": "Password",
-            "confirmPassword": "Confirm password",
-            "locale": "Language"
-        },
-        "localeOptions": {
-            "en": "English",
-            "nl": "Dutch"
-        },
-        "submit": "Create account",
-        "submitting": "Creating account…",
-        "success": "Account created. You can now sign in.",
-        "errors": {
-            "alreadyInitialized": "This system has already been set up.",
-            "generic": "Something went wrong. Please try again."
-        }
+        "nameLabel": "Full name",
+        "emailLabel": "Email",
+        "passwordLabel": "Password",
+        "confirmPasswordLabel": "Confirm password",
+        "languageLabel": "Language preference",
+        "submitButton": "Create owner account",
+        "successMessage": "Account created. You can now sign in."
     },
     "inviteComplete": {
         "title": "Complete your account",
-        "subtitle": "You have been invited to {{appName}}.",
-        "loading": "Validating your invitation…",
-        "fields": {
-            "email": "Email",
-            "name": "Full name",
-            "password": "Password",
-            "confirmPassword": "Confirm password"
-        },
-        "submit": "Complete setup",
-        "submitting": "Completing setup…",
-        "success": "Account setup complete. You can now sign in.",
-        "errors": {
-            "expired": "This invitation link has expired. Please request a new one.",
-            "used": "This invitation link has already been used.",
-            "notFound": "This invitation link is not valid.",
-            "noToken": "Invalid invitation link.",
-            "generic": "Something went wrong. Please try again."
-        }
+        "emailLabel": "Email",
+        "nameLabel": "Full name",
+        "passwordLabel": "Password",
+        "confirmPasswordLabel": "Confirm password",
+        "submitButton": "Complete setup",
+        "loadingMessage": "Validating your invitation…",
+        "invalidTokenMessage": "This invitation link has expired or is invalid.",
+        "requestNewInvitationLink": "Request a new invitation link",
+        "successMessage": "Account setup complete. You can now sign in."
     },
     "dashboard": {
         "title": "Dashboard",
@@ -83,8 +62,9 @@
     },
     "errors": {
         "network": "Unable to reach the server. Check your connection and try again.",
-        "accessDenied": "You do not have permission to view this page.",
-        "accessDeniedDetail": "This section requires the {{roles}} role.",
-        "sessionExpired": "Your session has expired. Please sign in again."
+        "generic": "Something went wrong. Please try again.",
+        "accessDenied": "You do not have permission to access this page.",
+        "setupRequired": "System setup required. Please initialize the system first.",
+        "invalidInvitationToken": "Invalid or expired invitation token."
     }
 }