Initial commit: notification-elements-ui library

Angular 19 component library for notifications & communication:
- ntf-bell: notification bell with badge count and shake animation
- ntf-feed / ntf-feed-item: real-time notification feed with grouping
- ntf-center: full notification center with category filter tabs
- ntf-inbox / ntf-inbox-item: two-column inbox with search and detail
- ntf-comment / ntf-thread: comment threads with replies and reactions
- ntf-mention-input: text input with @mention autocomplete
- ntf-empty-state: empty state placeholder
- ntf-item-def: custom template directive for notification items

Includes signal-based services, SCSS design tokens with dark mode,
utility functions, and full TypeScript type definitions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/ntf-item-def/ntf-item-def.directive.ts

@@ -0,0 +1,25 @@
+import { Directive, TemplateRef, inject } from '@angular/core';
+import type { NtfNotification } from '../../types/notification.types';
+
+/** Template context for custom notification item rendering */
+export interface NtfItemDefContext {
+  $implicit: NtfNotification;
+  notification: NtfNotification;
+  index: number;
+  isRead: boolean;
+}
+
+@Directive({
+  selector: 'ng-template[ntfItemDef]',
+  standalone: true,
+})
+export class NtfItemDefDirective {
+  readonly templateRef = inject(TemplateRef<NtfItemDefContext>);
+
+  static ngTemplateContextGuard(
+    _dir: NtfItemDefDirective,
+    ctx: unknown,
+  ): ctx is NtfItemDefContext {
+    return true;
+  }
+}