fix(unit-2-auth): Fix API endpoint paths and JSON casing for real backend

- Add /api/v1/ prefix to all Unit 2 API calls (Setup/status, Setup/owner,
  Invitation/validate, Invitation/complete) to match backend ApiPrefixConvention
- Fix setup status endpoint to POST /api/v1/Setup/owner (not /Setup)
- Handle PascalCase 'Initialized' response from .NET backend without camelCase policy
- Update all MSW mock handlers to match corrected /api/v1/ URL patterns
- Remove unused imports from RouteGuard.test.tsx

Root cause: backend uses ApiPrefixConvention('api/v1') but frontend calls
were missing the prefix, and .NET defaults to PascalCase JSON serialization.

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

frontend/src/api/useInvitation.ts

@@ -35,7 +35,7 @@ export function useValidateInvitation(token: string | null) {
             setIsPending(true);
             setError(null);
             try {
-                const url = `/Invitation/validate?token=${encodeURIComponent(token)}`;
+                const url = `/api/v1/Invitation/validate?token=${encodeURIComponent(token)}`;
                 const response = await api.get(url);
                 setData(response as InvitationValidationResponse);
             } catch (err: unknown) {
@@ -64,7 +64,7 @@ export function useCompleteInvitation() {
         setIsPending(true);
         setError(null);
         try {
-            const response = await api.post('/Invitation/complete', data);
+            const response = await api.post('/api/v1/Invitation/complete', data);
             return response as InvitationCompleteResponse;
         } catch (err: unknown) {
             const error = err instanceof Error ? err : new Error(String(err));