feat(design-system): add reusable Skeleton loading primitive

Standalone, OnPush, aria-hidden. Shapes text/circle/rect, configurable
width/height. Shimmer respects prefers-reduced-motion. Colors via
existing tenant CSS vars with fallbacks.

src/app/shared/ui/skeleton/skeleton.component.html

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+<!-- purely visual placeholder, no projected content -->

src/app/shared/ui/skeleton/skeleton.component.scss

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+:host {
+  display: block;
+  background: linear-gradient(
+    90deg,
+    var(--bg-secondary, #f4f4f5) 25%,
+    var(--border-color, #e4e4e7) 50%,
+    var(--bg-secondary, #f4f4f5) 75%
+  );
+  background-size: 200% 100%;
+  animation: app-skeleton-shimmer 1.4s ease-in-out infinite;
+}
+
+:host.app-skeleton--text {
+  height: 1em;
+  border-radius: var(--radius-sm, 4px);
+}
+
+:host.app-skeleton--circle {
+  border-radius: 50%;
+}
+
+:host.app-skeleton--rect {
+  border-radius: var(--radius-md, 6px);
+}
+
+@keyframes app-skeleton-shimmer {
+  0% {
+    background-position: 200% 0;
+  }
+  100% {
+    background-position: -200% 0;
+  }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  :host {
+    animation: none;
+    background: var(--bg-secondary, #f4f4f5);
+  }
+}

src/app/shared/ui/skeleton/skeleton.component.ts

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+import { ChangeDetectionStrategy, Component, input } from '@angular/core';
+
+export type SkeletonShape = 'text' | 'circle' | 'rect';
+
+@Component({
+  selector: 'app-skeleton',
+  standalone: true,
+  templateUrl: './skeleton.component.html',
+  styleUrl: './skeleton.component.scss',
+  changeDetection: ChangeDetectionStrategy.OnPush,
+  host: {
+    '[class.app-skeleton--text]': "shape() === 'text'",
+    '[class.app-skeleton--circle]': "shape() === 'circle'",
+    '[class.app-skeleton--rect]': "shape() === 'rect'",
+    '[style.width]': 'width()',
+    '[style.height]': 'height()',
+    'aria-hidden': 'true'
+  }
+})
+export class SkeletonComponent {
+  readonly shape = input<SkeletonShape>('text');
+  readonly width = input<string>('100%');
+  readonly height = input<string | null>(null);
+}