feat(design-system): add reusable Dialog primitive

Standalone, OnPush. Backdrop click and Escape close, focus trap with
return-focus on close, role=dialog/aria-modal. Sizes sm/md/lg. Colors/
radius/shadow/spacing via existing tenant CSS vars with fallbacks.

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

@@ -0,0 +1,33 @@
+@if (open()) {
+  <div class="app-dialog-backdrop" (click)="handleBackdropClick()">
+    <div
+      #panel
+      class="app-dialog-panel"
+      [class.app-dialog-panel--sm]="size() === 'sm'"
+      [class.app-dialog-panel--md]="size() === 'md'"
+      [class.app-dialog-panel--lg]="size() === 'lg'"
+      role="dialog"
+      aria-modal="true"
+      [attr.aria-label]="titleText()"
+      tabindex="-1"
+      (click)="$event.stopPropagation()"
+    >
+      @if (titleText()) {
+        <div class="app-dialog-panel__header">
+          <h2 class="app-dialog-panel__title">{{ titleText() }}</h2>
+          <button
+            type="button"
+            class="app-dialog-panel__close"
+            (click)="requestClose()"
+            aria-label="Close dialog"
+          >
+            &times;
+          </button>
+        </div>
+      }
+      <div class="app-dialog-panel__body">
+        <ng-content />
+      </div>
+    </div>
+  </div>
+}