Rename billing to payment throughout the codebase (#726)

* Rename billing to payment throughout the codebase

This change updates terminology from "billing" to "payment" to better
reflect that these are contributions/payments rather than bills.

Changes include:
- Rename BillingsController to PaymentsController
- Rename billing_email to payment_email
- Rename next_billing_date to next_payment_date
- Rename create_billing_portal_session_url to create_payment_portal_session_url
- Update routes from billing to payment
- Update all 12 locale files with new terminology
- Update views, helpers, and tests

* Update app/views/subscriptions/upgrade.html.erb

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Juan José Mata <jjmata@jjmata.com>

---------

Signed-off-by: Juan José Mata <jjmata@jjmata.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

app/helpers/settings_helper.rb

@@ -6,7 +6,7 @@ module SettingsHelper
     { name: "Preferences", path: :settings_preferences_path },
     { name: "Profile Info", path: :settings_profile_path },
     { name: "Security", path: :settings_security_path },
-    { name: "Billing", path: :settings_billing_path, condition: :not_self_hosted? },
+    { name: "Payment", path: :settings_payment_path, condition: :not_self_hosted? },
     # Transactions section
     { name: "Categories", path: :categories_path },
     { name: "Tags", path: :tags_path },