feat: add feature assignment to admin tier management UI

- Add slide-over panel for assigning features to subscription tiers
- Features grouped by category with select all/deselect all
- Add puzzle-piece icon button in tier table actions
- Add feature management methods to subscription-tiers.js
- Fix JS-006 by adding try/catch to init function

Documentation:
- Update feature-gating-system.md with Admin Tier Management UI section
- Update subscription-billing.md with tier management overview
- Add new admin user guide: subscription-tier-management.md
- Add guide to mkdocs.yml navigation

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

mkdocs.yml

@@ -211,6 +211,7 @@ nav:
       - User Management: guides/user-management.md
       - Product Management: guides/product-management.md
       - Inventory Management: guides/inventory-management.md
+      - Subscription Tier Management: guides/subscription-tier-management.md
       - Shop Setup: guides/shop-setup.md
       - CSV Import: guides/csv-import.md
       - Marketplace Integration: guides/marketplace-integration.md