feat: finish demo — Angular 22 + Material UI, role management, schema and Docker cleanup

Frontend
- Upgrade Angular 19 -> 22 (TypeScript 6, new control flow), drop unused
  packages (bootstrap, jquery, popper, ng-bootstrap, webpack)
- Move the UI to Angular Material: toolbar with user menu, tutorials list,
  add/edit forms, admin users table with role editing dialog, profile,
  home page with published tutorials, registration form, login dialog
- Guard routes: login required for add/edit/profile, admin role for /admin
- Remove the /login, Moderator and User pages, event bus, auth interceptor
  and other dead code

Backend
- Add PUT /api/users/{id}/roles (admin only) that keeps at least one admin
  and does not let an admin drop their own admin role
- Tighten security: read tutorials publicly, change them only when logged
  in, users/roles API for admins only; hide password hashes in JSON
- Remove unused Settings and Test endpoints, DTO mappers, commented code
- Flyway V2 aligns the schema with the entities (bigint ids, tutorials PK,
  identity ids); Hibernate now runs with ddl-auto: validate

Docker
- Three-stage Dockerfile (Node build, Gradle build, JRE runtime) with
  layer caching, non-root user and healthcheck; add .dockerignore
- docker-compose waits for a healthy Postgres, always rebuilds the image,
  no longer mounts the Docker socket

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

spring-angular-ui/src/app/components/roles-dialog/roles-dialog.component.html

@@ -0,0 +1,19 @@
+<h2 mat-dialog-title>Roles of {{ data.user.username }}</h2>
+
+<mat-dialog-content>
+  <div class="options">
+    @for (role of data.allRoles; track role.id) {
+      <mat-checkbox [checked]="isSelected(role.name!)" (change)="toggle(role.name!, $event.checked)">
+        {{ roleLabel(role.name) }}
+      </mat-checkbox>
+    }
+  </div>
+  @if (!result.length) {
+    <p class="hint">Select at least one role</p>
+  }
+</mat-dialog-content>
+
+<mat-dialog-actions align="end">
+  <button mat-button mat-dialog-close>Cancel</button>
+  <button mat-flat-button [mat-dialog-close]="result" [disabled]="!result.length">Save</button>
+</mat-dialog-actions>