docs: mark df-admin-interface as Laravel 13-transparent

[thekevinm]

Summary

This PR is part of the DreamFactory Laravel 11 -> 13 upgrade campaign
(Waves 0-5). df-admin-interface is the Angular 16 admin UI and has no
PHP/Laravel runtime coupling, so a code change is not required to maintain
compatibility with the L13 backend.

This is a docs-only alignment PR so the campaign has a tracked
shift/laravel-13 branch + PR for this package alongside the backend
packages.

Audit findings (see LARAVEL_13_COMPAT.md)

• All consumed API paths under /api/v2/... are stable across the L13
  upgrade — verified against df-core / df-user / df-system shift/laravel-13
  branches in Waves 0-1.
• Auth endpoints (/api/v2/system/admin/session, /api/v2/user/session,
  password / register flows) unchanged.
• The UI's HTTP interceptors consume df-core's custom error envelope
  ({ error: { message, code, context } }), not Laravel's default JSON
  error shape, so L11 -> L13 default-error format changes do not reach the UI.
• composer.json declares only dreamfactory/installer: * — no Laravel
  framework dependency.

Out of scope (flagged for future work)

• Angular 16 reached end-of-life in November 2024. A separate modernization
  track (Angular 17/18/19) is recommended, decoupled from this L13 upgrade.

Test plan

• No code changes — docs-only.
• composer.json audited: zero Laravel dep.
• src/app/shared/constants/urls.ts audited: all /api/v2/... paths
  stable in L13 backend.
• HTTP interceptors audited: custom df-core error envelope, not Laravel
  default JSON shape.
• Smoke verify UI still loads against the L13 backend once the host
  app's shift-173254 branch is deployable end-to-end (deferred to
  Wave 5 integration testing).