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>

docs/guides/subscription-tier-management.md

@@ -0,0 +1,135 @@
+# Subscription Tier Management
+
+This guide explains how to manage subscription tiers and assign features to them in the admin panel.
+
+## Accessing Tier Management
+
+Navigate to **Admin → Billing & Subscriptions → Subscription Tiers** or go directly to `/admin/subscription-tiers`.
+
+## Dashboard Overview
+
+The tier management page displays:
+
+### Stats Cards
+- **Total Tiers**: Number of configured subscription tiers
+- **Active Tiers**: Tiers currently available for subscription
+- **Public Tiers**: Tiers visible to vendors (excludes enterprise/custom)
+- **Est. MRR**: Estimated Monthly Recurring Revenue
+
+### Tier Table
+
+Each tier shows:
+
+| Column | Description |
+|--------|-------------|
+| # | Display order (affects pricing page order) |
+| Code | Unique identifier (e.g., `essential`, `professional`) |
+| Name | Display name shown to vendors |
+| Monthly | Monthly price in EUR |
+| Annual | Annual price in EUR (or `-` if not set) |
+| Orders/Mo | Monthly order limit (or `Unlimited`) |
+| Products | Product limit (or `Unlimited`) |
+| Team | Team member limit (or `Unlimited`) |
+| Features | Number of features assigned |
+| Status | Active, Private, or Inactive |
+| Actions | Edit Features, Edit, Activate/Deactivate |
+
+## Managing Tiers
+
+### Creating a New Tier
+
+1. Click **Create Tier** button
+2. Fill in the tier details:
+   - **Code**: Unique lowercase identifier (cannot be changed after creation)
+   - **Name**: Display name for the tier
+   - **Monthly Price**: Price in cents (e.g., 4900 for €49.00)
+   - **Annual Price**: Optional annual price in cents
+   - **Order Limit**: Leave empty for unlimited
+   - **Product Limit**: Leave empty for unlimited
+   - **Team Members**: Leave empty for unlimited
+   - **Display Order**: Controls sort order on pricing pages
+   - **Active**: Whether tier is available
+   - **Public**: Whether tier is visible to vendors
+3. Click **Create**
+
+### Editing a Tier
+
+1. Click the **pencil icon** on the tier row
+2. Modify the tier properties
+3. Click **Update**
+
+Note: The tier code cannot be changed after creation.
+
+### Activating/Deactivating Tiers
+
+- Click the **check-circle icon** to activate an inactive tier
+- Click the **x-circle icon** to deactivate an active tier
+
+Deactivating a tier:
+- Does not affect existing subscriptions
+- Hides the tier from new subscription selection
+- Can be reactivated at any time
+
+## Managing Features
+
+### Assigning Features to a Tier
+
+1. Click the **puzzle-piece icon** on the tier row
+2. A slide-over panel opens showing all available features
+3. Features are grouped by category:
+   - Analytics
+   - Product Management
+   - Order Management
+   - Marketing
+   - Support
+   - Integration
+   - Branding
+   - Team
+
+4. Check/uncheck features to include in the tier
+5. Use **Select all** or **Deselect all** per category for bulk actions
+6. The footer shows the total number of selected features
+7. Click **Save Features** to apply changes
+
+### Feature Categories
+
+| Category | Example Features |
+|----------|------------------|
+| Analytics | Basic Analytics, Analytics Dashboard, Custom Reports |
+| Product Management | Bulk Edit, Variants, Bundles, Inventory Alerts |
+| Order Management | Order Automation, Advanced Fulfillment, Multi-Warehouse |
+| Marketing | Discount Codes, Abandoned Cart, Email Marketing, Loyalty |
+| Support | Email Support, Priority Support, Phone Support, Dedicated Manager |
+| Integration | Basic API, Advanced API, Webhooks, Custom Integrations |
+| Branding | Theme Customization, Custom Domain, White Label |
+| Team | Team Management, Role Permissions, Audit Logs |
+
+## Best Practices
+
+### Tier Pricing Strategy
+
+1. **Essential**: Entry-level with basic features and limits
+2. **Professional**: Mid-tier with increased limits and key integrations
+3. **Business**: Full-featured for growing businesses
+4. **Enterprise**: Custom pricing with unlimited everything
+
+### Feature Assignment Tips
+
+- Start with fewer features in lower tiers
+- Ensure each upgrade tier adds meaningful value
+- Keep support features as upgrade incentives
+- API access typically belongs in Business+ tiers
+
+### Stripe Integration
+
+For each tier, you can optionally configure:
+- **Stripe Product ID**: Link to Stripe product
+- **Stripe Monthly Price ID**: Link to monthly price
+- **Stripe Annual Price ID**: Link to annual price
+
+These are required for automated billing via Stripe Checkout.
+
+## Related Documentation
+
+- [Subscription & Billing System](../features/subscription-billing.md) - Complete billing documentation
+- [Feature Gating System](../implementation/feature-gating-system.md) - Technical feature gating details