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/components/rt-variable-picker/rt-variable-picker.component.html

@@ -0,0 +1,43 @@
+<div class="rt-variable-picker">
+  <div class="rt-variable-picker__header">
+    <ui-input
+      size="sm"
+      placeholder="Search variables..."
+      [ngModel]="searchQuery()"
+      (ngModelChange)="searchQuery.set($event)"
+    />
+    <ui-button variant="ghost" size="sm" [iconOnly]="true" (click)="onClose()">
+      <ui-icon name="close" [size]="14" />
+    </ui-button>
+  </div>
+
+  <div class="rt-variable-picker__list">
+    @for (group of groupedVariables(); track group[0]) {
+      <div class="rt-variable-picker__category">
+        <div class="rt-variable-picker__category-label">{{ group[0] }}</div>
+        @for (variable of group[1]; track variable.key) {
+          <button
+            type="button"
+            class="rt-variable-picker__item"
+            [title]="variable.description || variable.key"
+            (click)="onSelect(variable)"
+          >
+            <div class="rt-variable-picker__item-content">
+              <span class="rt-variable-picker__item-label">{{ variable.label }}</span>
+              <span class="rt-variable-picker__item-key">{{ variable.key }}</span>
+            </div>
+            <ui-badge [variant]="getTypeBadgeVariant(variable.type)" size="sm">
+              {{ variable.type }}
+            </ui-badge>
+          </button>
+        }
+      </div>
+    }
+
+    @if (filteredVariables().length === 0) {
+      <div class="rt-variable-picker__empty">
+        No variables found
+      </div>
+    }
+  </div>
+</div>