Add Anthropic API, test environment, remove locale settings

Feature 1: Anthropic API Integration
- Add @anthropic-ai/sdk with adapter wrapping OpenAI-shaped interface
- Support Claude models (opus, sonnet, haiku) with extended thinking
- Auto-reset model on provider switch, JSON retry logic
- Add Claude model pricing to ai-usage tracker
- Update AI settings form with Anthropic provider option

Feature 2: Remove Locale Settings UI
- Strip Localization tab from admin settings
- Remove i18n settings from router inferCategory and getFeatureFlags
- Keep franc document language detection intact

Feature 3: Test Environment with Role Impersonation
- Add isTest field to User, Program, Project, Competition models
- Test environment service: create/teardown with realistic dummy data
- JWT-based impersonation for test users (@test.local emails)
- Impersonation banner with quick-switch between test roles
- Test environment panel in admin settings (SUPER_ADMIN only)
- Email redirect: @test.local emails routed to admin with [TEST] prefix
- Complete data isolation: 45+ isTest:false filters across platform
  - All global queries on User/Project/Program/Competition
  - AI services blocked from processing test data
  - Cron jobs skip test rounds/users
  - Analytics/exports exclude test data
  - Admin layout/pickers hide test programs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/(admin)/admin/competitions/[competitionId]/assignments/page.tsx

@@ -68,8 +68,19 @@ export default function AssignmentsDashboardPage() {
 
   if (!competition) {
     return (
-    <div className="container mx-auto space-y-6 p-4 sm:p-6">
-        <p>Competition not found</p>
+      <div className="container mx-auto space-y-6 p-4 sm:p-6">
+        <Card>
+          <CardContent className="flex flex-col items-center justify-center py-12">
+            <p className="font-medium">Competition not found</p>
+            <p className="text-sm text-muted-foreground mt-1">
+              The requested competition does not exist or you don&apos;t have access.
+            </p>
+            <Button variant="outline" className="mt-4" onClick={() => router.back()}>
+              <ArrowLeft className="mr-2 h-4 w-4" />
+              Go Back
+            </Button>
+          </CardContent>
+        </Card>
       </div>
     )
   }