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>

static/vendor/js/billing.js

@@ -3,10 +3,11 @@
 
 const billingLog = window.LogConfig?.createLogger('BILLING') || console;
 
-function billingData() {
+function vendorBilling() {
     return {
         // Inherit base data (dark mode, sidebar, vendor info, etc.)
         ...data(),
+        currentPage: 'billing',
 
         // State
         loading: true,