fix: rename billingData to vendorBilling for JS-003 rule compliance

The JS-003 architecture rule only matches function names starting with
admin*, vendor*, or shop*. Renamed billingData() to vendorBilling() to
follow the vendor JS naming convention and ensure future violations are
caught by architecture validation.

Also added currentPage: 'billing' for JS-004 compliance.

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

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

app/templates/vendor/billing.html

@@ -3,7 +3,7 @@
 
 {% block title %}Billing & Subscription{% endblock %}
 
-{% block alpine_data %}billingData(){% endblock %}
+{% block alpine_data %}vendorBilling(){% endblock %}
 
 {% block content %}
 <div class="flex items-center justify-between my-6">