feat: add EnvBanner component for non-production environments

Standalone component that renders a small banner whenever
environment.production is false, so the local dev build is never
mistaken for the real app.

myteamwallet_frontend_modern/src/app/shared/env-banner/env-banner.html

@@ -0,0 +1,3 @@
+@if (showBanner) {
+  <div class="env-banner" role="status">⚠ Entwicklungsumgebung</div>
+}

myteamwallet_frontend_modern/src/app/shared/env-banner/env-banner.scss

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+// Höhe muss mit ENV_BANNER_HEIGHT_PX in env-banner.ts übereinstimmen.
+.env-banner {
+  height: 28px;
+  flex-shrink: 0;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: #f4a300;
+  color: #20251f;
+  font-weight: 700;
+  font-size: 0.75rem;
+  letter-spacing: 0.04em;
+  text-transform: uppercase;
+}

myteamwallet_frontend_modern/src/app/shared/env-banner/env-banner.spec.ts

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+import { TestBed } from '@angular/core/testing';
+import { EnvBanner } from './env-banner';
+import { environment } from '../../../environments/environment';
+
+describe('EnvBanner', () => {
+  const originalProduction = environment.production;
+
+  afterEach(() => {
+    environment.production = originalProduction;
+  });
+
+  it('shows the environment banner outside production', async () => {
+    environment.production = false;
+    await TestBed.configureTestingModule({ imports: [EnvBanner] }).compileComponents();
+    const fixture = TestBed.createComponent(EnvBanner);
+    fixture.detectChanges();
+
+    const element = fixture.nativeElement.querySelector('.env-banner');
+    expect(element?.textContent).toContain('Entwicklungsumgebung');
+  });
+
+  it('renders nothing in production', async () => {
+    environment.production = true;
+    await TestBed.configureTestingModule({ imports: [EnvBanner] }).compileComponents();
+    const fixture = TestBed.createComponent(EnvBanner);
+    fixture.detectChanges();
+
+    const element = fixture.nativeElement.querySelector('.env-banner');
+    expect(element).toBeNull();
+  });
+});

myteamwallet_frontend_modern/src/app/shared/env-banner/env-banner.ts

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+import { Component } from '@angular/core';
+import { environment } from '../../../environments/environment';
+
+export const ENV_BANNER_HEIGHT_PX = 28;
+
+@Component({
+  selector: 'app-env-banner',
+  templateUrl: './env-banner.html',
+  styleUrl: './env-banner.scss',
+})
+export class EnvBanner {
+  protected readonly showBanner = !environment.production;
+}