feat(design-system): add reusable Button primitive

Sprint 6 (pulled forward as prerequisite to Sprint 3). First Design
System component: app-button, standalone/OnPush, variants
(primary/secondary/ghost/danger), sizes (sm/md/lg), loading + disabled
states, focus-visible ring, reduced-motion-aware spinner. Colors/radius/
shadow/spacing consumed via existing tenant-driven CSS vars
(--primary-color, --bg-primary, --radius-md, --shadow-sm, --space-*) —
no new hardcoded palette, fully compatible with bootstrap JSON theming
and Project Editor live preview.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

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+<button
+  class="app-button"
+  [class.app-button--sm]="size() === 'sm'"
+  [class.app-button--md]="size() === 'md'"
+  [class.app-button--lg]="size() === 'lg'"
+  [class.app-button--primary]="variant() === 'primary'"
+  [class.app-button--secondary]="variant() === 'secondary'"
+  [class.app-button--ghost]="variant() === 'ghost'"
+  [class.app-button--danger]="variant() === 'danger'"
+  [class.app-button--loading]="loading()"
+  [type]="type()"
+  [disabled]="disabled() || loading()"
+  [attr.aria-busy]="loading() || null"
+>
+  @if (loading()) {
+    <span class="app-button__spinner" aria-hidden="true"></span>
+  }
+  <span class="app-button__label"><ng-content /></span>
+</button>

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

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+:host {
+  display: inline-block;
+}
+
+:host.app-button-host--full-width {
+  display: block;
+  width: 100%;
+}
+
+.app-button {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  gap: var(--space-xs, 0.25rem);
+  width: 100%;
+  border: 1px solid transparent;
+  border-radius: var(--radius-md, 6px);
+  font-family: inherit;
+  font-weight: 600;
+  line-height: 1;
+  cursor: pointer;
+  transition: background-color var(--transition-fast, 120ms ease),
+    border-color var(--transition-fast, 120ms ease),
+    box-shadow var(--transition-fast, 120ms ease),
+    opacity var(--transition-fast, 120ms ease);
+
+  &:focus-visible {
+    outline: 2px solid var(--primary-color, #2f6e5d);
+    outline-offset: 2px;
+  }
+
+  &:disabled {
+    cursor: not-allowed;
+    opacity: 0.6;
+  }
+}
+
+.app-button--sm {
+  height: 32px;
+  padding: 0 var(--space-sm, 0.5rem);
+  font-size: 0.8125rem;
+}
+
+.app-button--md {
+  height: 40px;
+  padding: 0 var(--space-md, 1rem);
+  font-size: 0.9375rem;
+}
+
+.app-button--lg {
+  height: 48px;
+  padding: 0 var(--space-lg, 1.5rem);
+  font-size: 1rem;
+}
+
+.app-button--primary {
+  background: var(--primary-color, #2f6e5d);
+  border-color: var(--primary-color, #2f6e5d);
+  color: var(--bg-primary, #fff);
+
+  &:hover:not(:disabled) {
+    box-shadow: var(--shadow-sm, 0 1px 2px rgba(0, 0, 0, 0.08));
+  }
+}
+
+.app-button--secondary {
+  background: var(--bg-secondary, #f4f4f5);
+  border-color: var(--border-color, #e4e4e7);
+  color: var(--text-primary, #1f322d);
+
+  &:hover:not(:disabled) {
+    border-color: var(--primary-color, #2f6e5d);
+  }
+}
+
+.app-button--ghost {
+  background: transparent;
+  border-color: transparent;
+  color: var(--text-primary, #1f322d);
+
+  &:hover:not(:disabled) {
+    background: var(--bg-secondary, #f4f4f5);
+  }
+}
+
+.app-button--danger {
+  background: var(--error-color, #c0392b);
+  border-color: var(--error-color, #c0392b);
+  color: var(--bg-primary, #fff);
+
+  &:hover:not(:disabled) {
+    box-shadow: var(--shadow-sm, 0 1px 2px rgba(0, 0, 0, 0.08));
+  }
+}
+
+.app-button__spinner {
+  width: 1em;
+  height: 1em;
+  border: 2px solid currentColor;
+  border-right-color: transparent;
+  border-radius: 50%;
+  animation: app-button-spin 0.6s linear infinite;
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .app-button {
+    transition: none;
+  }
+
+  .app-button__spinner {
+    animation-duration: 1.2s;
+  }
+}
+
+@keyframes app-button-spin {
+  to {
+    transform: rotate(360deg);
+  }
+}

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

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+import { ChangeDetectionStrategy, Component, input } from '@angular/core';
+
+export type ButtonVariant = 'primary' | 'secondary' | 'ghost' | 'danger';
+export type ButtonSize = 'sm' | 'md' | 'lg';
+
+@Component({
+  selector: 'app-button',
+  standalone: true,
+  templateUrl: './button.component.html',
+  styleUrl: './button.component.scss',
+  changeDetection: ChangeDetectionStrategy.OnPush,
+  host: {
+    '[class.app-button-host--full-width]': 'fullWidth()'
+  }
+})
+export class ButtonComponent {
+  readonly variant = input<ButtonVariant>('primary');
+  readonly size = input<ButtonSize>('md');
+  readonly type = input<'button' | 'submit' | 'reset'>('button');
+  readonly disabled = input(false);
+  readonly loading = input(false);
+  readonly fullWidth = input(false);
+}