Refactor theme templates to Single Directory Components (SDC)

Summary

Migrate the openintranet_theme from traditional Twig templates + SCSS to Drupal 11 Single Directory Components (SDC) architecture.

Current State

• ~80 Twig templates + 44 SCSS files
• 1 existing SDC component (events-nav-tabs)
• Reusable UI patterns are duplicated across templates with no encapsulation

Goal

• Extract reusable UI elements as SDC components (auto-discovery, lazy loading CSS/JS, prop validation)
• Reduce global CSS by ~30%
• All new UI elements created as SDC going forward

Planned Components

Priority 1 — Reusable UI: event-item, news-card, user-card, bookmark-flag, reaction-widget, pager, search-result

Priority 2 — Blocks: banner-block, quick-action, sidebar-block, knowledge-nav

Phases

1. Infrastructure & validation (enforce_sdc_schemas, update skills)
2. Reusable UI elements (7 components)
3. Block components (4 components)
4. Cleanup (remove duplicate SCSS, regression testing)