Updated Billing_System_Final_Plan.md: Billing is now a top-level sidebar module at /platform/services/billing/ (not under Settings). Includes full module registration (domain_controlpanel, domain_tblsite_menu, domain_purchased_modules), pagedata.php with 8 pages, file structure, URL routing table, and detailed UI specs for all 7 tabs (Overview, Invoices, Invoice Detail, Subscription, Usage, Payment Methods, Payment History, Reports).