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/views/settings/_settings_nav.html.erb

@@ -8,7 +8,7 @@ nav_sections = [
       { label: t(".preferences_label"), path: settings_preferences_path, icon: "bolt" },
       { label: t(".profile_label"), path: settings_profile_path, icon: "circle-user" },
       { label: t(".security_label"), path: settings_security_path, icon: "shield-check" },
-      { label: t(".billing_label"), path: settings_billing_path, icon: "circle-dollar-sign", if: !self_hosted? }
+      { label: t(".payment_label"), path: settings_payment_path, icon: "circle-dollar-sign", if: !self_hosted? }
     ]
   },
   {