Add submit button component and style it.

frontend/src/app/components/submit-button/submit-button.component.html

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+<button type="submit" [class]="color" [disabled]="disabled || requestPending" (click)="click.emit()">
+    @if (requestPending) {
+        <mat-spinner class="spinner {{color}}" [diameter]="25"></mat-spinner>
+    }
+    <span>
+        {{ label }}
+    </span>
+</button>

frontend/src/app/components/submit-button/submit-button.component.scss

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+button {
+    padding: .8em 1.2em;
+    border-radius: 10em;
+    border: none;
+    background-color: #3f51b5;
+    color: white;
+    transition: background-color .2s ease-in-out;
+    position: relative;
+
+    &:hover {
+        background-color: #5b6ed8;
+        cursor: pointer;
+    }
+
+    &.secondary {
+        color: #3f51b5;
+        background-color: white;
+
+        &:hover {
+            background-color: #f2f4ff;
+            cursor: pointer;
+        }
+    }
+
+    &:disabled {
+        background-color: #6d7ac5;
+        cursor: not-allowed;
+    }
+
+    mat-spinner {
+        position: absolute;
+        top: calc(50% - 12px);
+        left: calc(50% - 12px);
+    }
+}
+
+:host ::ng-deep .spinner circle {
+    stroke: white;
+}
+
+:host ::ng-deep .spinner.secondary circle {
+    stroke: #3f51b5;
+}

frontend/src/app/components/submit-button/submit-button.component.ts

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+import { CommonModule } from "@angular/common";
+import { Component, EventEmitter, Input, Output } from "@angular/core";
+import { MatProgressSpinnerModule } from "@angular/material/progress-spinner";
+
+
+@Component({
+    selector: 'app-submit-button',
+    standalone: true,
+    imports: [MatProgressSpinnerModule, CommonModule],
+    templateUrl: 'submit-button.component.html',
+    styleUrl: 'submit-button.component.scss'
+})
+export class SubmitButtonComponent {
+    @Input() requestPending: boolean = false;
+    @Input() label: string = '';
+    @Input() disabled: boolean = false;
+    @Input() color?: 'secondary';
+    @Output() click = new EventEmitter<void>();
+  }
+