v0.0.7 ~ Complete invoice settings functionality

[roncodes]

Summary

• make payment_terms_days the canonical invoice terms setting while preserving due_date_offset_days as a legacy alias
• apply payment terms defaults to manual and backend invoice creation
• wire auto_send_on_creation through the shared invoice send flow for manual and order-generated invoices

Validation

• php -l server/src/Http/Controllers/Internal/v1/SettingController.php
• php -l server/src/Models/Invoice.php
• php -l server/src/Services/InvoiceService.php
• php -l server/src/Http/Controllers/Internal/v1/InvoiceController.php
• php -l server/tests/Feature.php
• ./node_modules/.bin/eslint addon/controllers/billing/invoices/index/new.js
• git diff --check

Pest was not run because pest/vendor is not installed in this checkout.