feat: initial rich-text-elements-ui library implementation

TipTap-powered rich text editing library with WYSIWYG editor, markdown editor,
template editor, collaboration support (Yjs), mentions, track changes,
comments, code blocks, and table insertion.

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

src/services/history.service.ts

@@ -0,0 +1,25 @@
+import { Injectable } from '@angular/core';
+import type { Editor } from '@tiptap/core';
+
+@Injectable()
+export class HistoryService {
+  /** Undo the last action */
+  undo(editor: Editor): void {
+    editor.chain().focus().undo().run();
+  }
+
+  /** Redo the last undone action */
+  redo(editor: Editor): void {
+    editor.chain().focus().redo().run();
+  }
+
+  /** Check if undo is available */
+  canUndo(editor: Editor): boolean {
+    return editor.can().undo();
+  }
+
+  /** Check if redo is available */
+  canRedo(editor: Editor): boolean {
+    return editor.can().redo();
+  }
+}