Add comprehensive component library expansion with new UI components

This commit includes multiple new components and improvements to the UI essentials library:

## New Components Added:
- **Accordion**: Expandable/collapsible content panels with full accessibility
- **Alert**: Notification and messaging component with variants and dismiss functionality
- **Popover**: Floating content containers with positioning and trigger options
- **Timeline**: Chronological event display with customizable styling
- **Tree View**: Hierarchical data display with expand/collapse functionality
- **Toast**: Notification component (previously committed, includes style refinements)

## Component Features:
- Full TypeScript implementation with Angular 19+ patterns
- Comprehensive SCSS styling using semantic design tokens exclusively
- Multiple size variants (sm, md, lg) and color variants (primary, success, warning, danger, info)
- Accessibility support with ARIA attributes and keyboard navigation
- Responsive design with mobile-first approach
- Interactive demos showcasing all component features
- Integration with existing design system and routing

## Demo Applications:
- Created comprehensive demo components for each new component
- Interactive examples with live code demonstrations
- Integrated into main demo application routing and navigation

## Documentation:
- Added COMPONENT_CREATION_TEMPLATE.md with detailed guidelines
- Comprehensive component creation patterns and best practices
- Design token usage guidelines and validation rules

## Code Quality:
- Follows established Angular and SCSS conventions
- Uses only verified semantic design tokens
- Maintains consistency with existing component architecture
- Comprehensive error handling and edge case management

🤖 Generated with [Claude Code](https://claude.ai/code)

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

projects/demo-ui-essentials/src/app/demos/tree-view-demo/tree-view-demo.component.scss

@@ -0,0 +1,116 @@
+@use '../../../../../shared-ui/src/styles/semantic/index' as *;
+
+.demo-container {
+  padding: $semantic-spacing-component-lg;
+}
+
+.demo-section {
+  margin-bottom: $semantic-spacing-layout-section-lg;
+  
+  h3 {
+    font-family: map-get($semantic-typography-heading-h3, font-family);
+    font-size: map-get($semantic-typography-heading-h3, font-size);
+    font-weight: map-get($semantic-typography-heading-h3, font-weight);
+    line-height: map-get($semantic-typography-heading-h3, line-height);
+    color: $semantic-color-text-primary;
+    margin-bottom: $semantic-spacing-content-heading;
+    border-bottom: $semantic-border-width-1 solid $semantic-color-border-subtle;
+    padding-bottom: $semantic-spacing-component-sm;
+  }
+  
+  h4 {
+    font-family: map-get($semantic-typography-heading-h4, font-family);
+    font-size: map-get($semantic-typography-heading-h4, font-size);
+    font-weight: map-get($semantic-typography-heading-h4, font-weight);
+    line-height: map-get($semantic-typography-heading-h4, line-height);
+    color: $semantic-color-text-primary;
+    margin-bottom: $semantic-spacing-component-sm;
+  }
+}
+
+.demo-row {
+  display: flex;
+  gap: $semantic-spacing-layout-section-md;
+  flex-wrap: wrap;
+}
+
+.demo-column {
+  flex: 1;
+  min-width: 300px;
+  
+  .ui-tree-view {
+    max-height: 300px;
+    overflow-y: auto;
+  }
+}
+
+.demo-controls {
+  display: flex;
+  gap: $semantic-spacing-component-sm;
+  margin-bottom: $semantic-spacing-component-md;
+  flex-wrap: wrap;
+}
+
+.demo-button {
+  padding: $semantic-spacing-interactive-button-padding-y $semantic-spacing-interactive-button-padding-x;
+  border: $semantic-border-width-1 solid $semantic-color-border-primary;
+  border-radius: $semantic-border-button-radius;
+  background: $semantic-color-surface-primary;
+  color: $semantic-color-text-primary;
+  cursor: pointer;
+  transition: all $semantic-motion-duration-fast $semantic-motion-easing-ease;
+  
+  font-family: map-get($semantic-typography-button-medium, font-family);
+  font-size: map-get($semantic-typography-button-medium, font-size);
+  font-weight: map-get($semantic-typography-button-medium, font-weight);
+  line-height: map-get($semantic-typography-button-medium, line-height);
+  
+  &:hover {
+    background: $semantic-color-surface-secondary;
+    box-shadow: $semantic-shadow-button-hover;
+  }
+  
+  &:focus-visible {
+    outline: 2px solid $semantic-color-focus;
+    outline-offset: 2px;
+  }
+  
+  &:active {
+    box-shadow: $semantic-shadow-button-rest;
+  }
+}
+
+.demo-info {
+  margin-top: $semantic-spacing-component-md;
+  padding: $semantic-spacing-component-sm;
+  background: $semantic-color-surface-secondary;
+  border-radius: $semantic-border-radius-sm;
+  border-left: 4px solid $semantic-color-primary;
+  
+  font-family: map-get($semantic-typography-body-small, font-family);
+  font-size: map-get($semantic-typography-body-small, font-size);
+  font-weight: map-get($semantic-typography-body-small, font-weight);
+  line-height: map-get($semantic-typography-body-small, line-height);
+  color: $semantic-color-text-secondary;
+  
+  strong {
+    color: $semantic-color-text-primary;
+    font-weight: $semantic-typography-font-weight-semibold;
+  }
+}
+
+// Responsive Design
+@media (max-width: $semantic-breakpoint-md - 1) {
+  .demo-row {
+    flex-direction: column;
+    gap: $semantic-spacing-component-md;
+  }
+  
+  .demo-column {
+    min-width: unset;
+  }
+  
+  .demo-controls {
+    justify-content: center;
+  }
+}