feat: add vendor notifications and analytics pages (Phase 3)

New pages:
- Notifications center: view, mark read, delete, settings modal
- Analytics: period selector, stats overview, feature-gated advanced metrics

Changes:
- Add routes for /vendor/{code}/notifications and /analytics
- Create notifications.js and analytics.js with full functionality
- Create notifications.html and analytics.html templates
- Update sidebar with Analytics link and Notifications in Customers section
- Update vendor-frontend-parity-plan.md to mark Phase 3 complete

Vendor frontend now at ~95% parity with admin.

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

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

app/templates/vendor/partials/sidebar.html

@@ -72,6 +72,7 @@
     <!-- Dashboard (always visible) -->
     <ul class="mt-6">
         {{ menu_item('dashboard', 'dashboard', 'home', 'Dashboard') }}
+        {{ menu_item('analytics', 'analytics', 'chart-bar', 'Analytics') }}
     </ul>
 
     <!-- Products & Inventory Section -->
@@ -95,6 +96,7 @@
     {% call section_content('customers') %}
         {{ menu_item('customers', 'customers', 'user-group', 'All Customers') }}
         {{ menu_item('messages', 'messages', 'chat-bubble-left-right', 'Messages') }}
+        {{ menu_item('notifications', 'notifications', 'bell', 'Notifications') }}
     {% endcall %}
 
     <!-- Shop & Content Section -->