fix(admin-auth): reuse exact same QR/session API and component for admin login

- Removed invented adminAuthApiUrl endpoint and separate AdminLoginComponent.
  Admin login now uses the exact same Telegram session backend
  (TelegramSessionApiService, {authApiUrl}/users/sessions) and the exact
  same TelegramLoginComponent (mode="customer" | "admin" input) as customer
  login - only the storage (cookie/localStorage/signals) stays separate.
- Extracted the shared HTTP+normalization logic from AuthService into
  TelegramSessionApiService so both AuthService and AdminAuthService call it
  instead of duplicating request/parsing code.
- Documented the resulting backend gap in docs/Project-Editor.md: since the
  session API has no concept of "admin", server-side role enforcement is
  required when admin API calls are made - the frontend only decides where
  to store the session, not whether the user is actually an admin.

src/app/app.html

@@ -25,5 +25,5 @@
     <div class="footer-placeholder" aria-hidden="true"></div>
   }
   <!-- <app-telegram-login /> -->
-  <app-admin-login />
+  <app-telegram-login mode="admin" />
 }