docs(auth): add docs/AUTH.md, drop unused RouterLink import

Sequence diagrams, JWT claims, API contracts, error responses,
permission model, refresh lifecycle, security considerations, cutover
plan. ng build passes clean (pre-existing bundle-budget warnings
unrelated to this change).

src/app/core/auth/pages/auth-error-page.component.ts

@@ -1,6 +1,6 @@
 import { ChangeDetectionStrategy, Component, computed, inject } from '@angular/core';
 import { toSignal } from '@angular/core/rxjs-interop';
-import { ActivatedRoute, Router, RouterLink } from '@angular/router';
+import { ActivatedRoute, Router } from '@angular/router';
 import { map } from 'rxjs';
 import { ButtonComponent } from '../../../shared/ui/button/button.component';
 import { EmptyStateComponent } from '../../../shared/ui/empty-state/empty-state.component';
@@ -48,7 +48,7 @@ const COPY: Record<AuthErrorCode, AuthErrorCopy> = {
 @Component({
   selector: 'app-auth-error-page',
   standalone: true,
-  imports: [EmptyStateComponent, ButtonComponent, RouterLink],
+  imports: [EmptyStateComponent, ButtonComponent],
   templateUrl: './auth-error-page.component.html',
   styleUrl: './auth-error-page.component.scss',
   changeDetection: ChangeDetectionStrategy.OnPush