refactor: remove security-related hooks and pages

- Deleted `useSecurity.ts` hook which managed authentication users, providers, and policies.
- Removed `Policies.tsx`, `Providers.tsx`, and `Users.tsx` pages that utilized the above hook.
- Cleaned up the `index.tsx` file in the Security section to remove references to the deleted pages.
- Updated mock data by removing unused properties related to forward authentication.

frontend/src/hooks/__tests__/useProxyHosts.test.tsx

@@ -26,8 +26,6 @@ const createMockHost = (overrides: Partial<api.ProxyHost> = {}): api.ProxyHost =
   hsts_subdomains: false,
   block_exploits: false,
   websocket_support: false,
-  forward_auth_enabled: false,
-  forward_auth_bypass: '',
   locations: [],
   enabled: true,
   created_at: '2025-01-01T00:00:00Z',