feat: finish demo — port demo-monolith cleanup, separate UI and API containers

Frontend
- Bring spring-angular-ui back under version control with the Angular 22 +
  Material UI from demo-monolith; stop committing the built static/ folder
- Serve the UI from nginx, which proxies /api to the backend so the JWT
  cookie works on a single origin, and falls back to index.html for routes

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; Hibernate now runs with
  ddl-auto: validate
- JWT lifetime matches the 24h cookie; fix the cookie name typo
- Keep CORS only for calling the API directly (ng serve without proxy)

Docker
- Backend Dockerfile: Gradle build + JRE runtime with layer caching,
  non-root user and healthcheck; add .dockerignore
- UI Dockerfile: Node 24 build + nginx with healthcheck
- docker-compose runs its own Postgres, starts services in health order
  and always rebuilds images

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

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

@@ -1,29 +1,51 @@
-<h1 mat-dialog-title>Hi </h1>
-<div mat-dialog-content>
-  <p>User name</p>
-  <mat-form-field class="fillField">
-    <input matInput [(ngModel)]="data.username">
-  </mat-form-field>
-  <p>Password</p>
-  <mat-form-field class="fillField">
-    <mat-label>Enter your password</mat-label>
-    <input matInput
-           [type]="hide() ? 'password' : 'text'"
-           [(ngModel)]="data.password"
-    />
-    <button
-      mat-icon-button
-      matSuffix
-      (click)="clickEvent($event)"
-      [attr.aria-label]="'Hide password'"
-      [attr.aria-pressed]="hide()"
-    >
-      <mat-icon>{{hide() ? 'visibility_off' : 'visibility'}}</mat-icon>
-    </button>
-  </mat-form-field>
-</div>
+<form #f="ngForm" (ngSubmit)="f.valid && signIn()">
+  <div class="heading">
+    <span class="heading-icon"><mat-icon>login</mat-icon></span>
+    <h2 class="title">Sign in</h2>
+    <p class="subtitle">Welcome back! Enter your details to continue.</p>
+  </div>
 
-<div mat-dialog-actions>
-  <button mat-button mat-dialog-close (click)="onNoClick()">No Thanks</button>
-  <button mat-button [mat-dialog-close]="data" cdkFocusInitial>Ok</button>
-</div>
+  <mat-dialog-content>
+    <mat-form-field appearance="outline" class="full-width">
+      <mat-label>Username</mat-label>
+      <mat-icon matPrefix>person</mat-icon>
+      <input matInput name="username" autocomplete="username" required cdkFocusInitial [(ngModel)]="username" />
+      <mat-error>Username is required</mat-error>
+    </mat-form-field>
+
+    <mat-form-field appearance="outline" class="full-width">
+      <mat-label>Password</mat-label>
+      <mat-icon matPrefix>lock</mat-icon>
+      <input matInput name="password" autocomplete="current-password" required
+             [type]="hidePassword() ? 'password' : 'text'" [(ngModel)]="password" />
+      <button matIconButton matSuffix type="button" (click)="togglePassword($event)"
+              [attr.aria-label]="hidePassword() ? 'Show password' : 'Hide password'"
+              [attr.aria-pressed]="!hidePassword()">
+        <mat-icon>{{ hidePassword() ? 'visibility' : 'visibility_off' }}</mat-icon>
+      </button>
+      <mat-error>Password is required</mat-error>
+    </mat-form-field>
+
+    @if (errorMessage()) {
+      <div class="error-box" role="alert">
+        <mat-icon>error</mat-icon>
+        <span>{{ errorMessage() }}</span>
+      </div>
+    }
+
+    <p class="demo-hint">
+      <mat-icon>info</mat-icon>
+      <span>Demo: <code>Admin</code> / <code>pwd123</code></span>
+    </p>
+  </mat-dialog-content>
+
+  <mat-dialog-actions class="actions">
+    <button mat-flat-button type="submit" class="full-width" [disabled]="submitting()">
+      {{ submitting() ? 'Signing in…' : 'Sign in' }}
+    </button>
+    <p class="register">
+      No account yet?
+      <a routerLink="/register" mat-dialog-close>Create one</a>
+    </p>
+  </mat-dialog-actions>
+</form>