feat: Datafields page, merge fee types into membership_fee_settings, sidebar

- Add /admin/datafields (DatafieldsLive) for member and custom field config
- Remove Memberdata block from GlobalSettingsLive
- Router: drop /membership_fee_types, add new_fee_type and edit_fee_type under membership_fee_settings
- MembershipFeeSettingsLive: fee types table, collapsible examples; Index links updated
- PagePaths: admin_datafields, admin_import; remove membership_fee_types
- Sidebar: order and labels (Basic settings, Datafields, Membership fee settings, Import, Users, Roles)
- Gettext: German translations for sidebar and OIDC
- Tests: datafields and fee routes, permission and form tests updated

lib/mv_web/router.ex

@@ -68,16 +68,13 @@ defmodule MvWeb.Router do
 
       live "/settings", GlobalSettingsLive
 
-      # Membership Fee Settings
+      # Membership Fee Settings (includes fee types list; new/edit under sub-routes)
       live "/membership_fee_settings", MembershipFeeSettingsLive
-
-      # Membership Fee Types Management
-      live "/membership_fee_types", MembershipFeeTypeLive.Index, :index
+      live "/membership_fee_settings/new_fee_type", MembershipFeeTypeLive.Form, :new
+      live "/membership_fee_settings/:id/edit_fee_type", MembershipFeeTypeLive.Form, :edit
 
       # Statistics
       live "/statistics", StatisticsLive, :index
-      live "/membership_fee_types/new", MembershipFeeTypeLive.Form, :new
-      live "/membership_fee_types/:id/edit", MembershipFeeTypeLive.Form, :edit
 
       # Groups Management
       live "/groups", GroupLive.Index, :index
@@ -91,6 +88,9 @@ defmodule MvWeb.Router do
       live "/admin/roles/:id", RoleLive.Show, :show
       live "/admin/roles/:id/edit", RoleLive.Form, :edit
 
+      # Datafields (member fields + custom fields)
+      live "/admin/datafields", DatafieldsLive
+
       # Import (Admin only)
       live "/admin/import", ImportLive