chore: clean up sora_client_enabled and purchase_subscription residuals

- Remove sora_client_enabled from PublicSettings type and store defaults
- Remove purchase_subscription form defaults and save payload from SettingsView
- Remove dead 'data' tab type from SettingsTab union

frontend/src/stores/app.ts

@@ -338,7 +338,6 @@ export const useAppStore = defineStore('app', () => {
         linuxdo_oauth_enabled: false,
         oidc_oauth_enabled: false,
         oidc_oauth_provider_name: 'OIDC',
-        sora_client_enabled: false,
         backend_mode_enabled: false,
         version: siteVersion.value
       }

frontend/src/types/index.ts

@@ -112,7 +112,6 @@ export interface PublicSettings {
   linuxdo_oauth_enabled: boolean
   oidc_oauth_enabled: boolean
   oidc_oauth_provider_name: string
-  sora_client_enabled: boolean
   backend_mode_enabled: boolean
   version: string
 }

frontend/src/views/admin/SettingsView.vue

@@ -2540,7 +2540,7 @@ const { t } = useI18n()
 const appStore = useAppStore()
 const adminSettingsStore = useAdminSettingsStore()
 
-type SettingsTab = 'general' | 'security' | 'users' | 'gateway' | 'payment' | 'email' | 'backup' | 'data'
+type SettingsTab = 'general' | 'security' | 'users' | 'gateway' | 'payment' | 'email' | 'backup'
 const activeTab = ref<SettingsTab>('general')
 const settingsTabs = [
   { key: 'general'  as SettingsTab, icon: 'home'   as const },
@@ -2660,8 +2660,6 @@ const form = reactive<SettingsForm>({
   backend_mode_enabled: false,
   hide_ccs_import_button: false,
   payment_enabled: false,  payment_min_amount: 1,  payment_max_amount: 10000,  payment_daily_limit: 50000,  payment_max_pending_orders: 3,  payment_order_timeout_minutes: 30,  payment_balance_disabled: false,  payment_enabled_types: [],  payment_help_image_url: '',  payment_help_text: '',  payment_product_name_prefix: '',  payment_product_name_suffix: '',  payment_load_balance_strategy: 'round-robin',  payment_cancel_rate_limit_enabled: false,  payment_cancel_rate_limit_max: 10,  payment_cancel_rate_limit_window: 1,  payment_cancel_rate_limit_unit: 'day',  payment_cancel_rate_limit_window_mode: 'rolling',
-  purchase_subscription_enabled: false,
-  purchase_subscription_url: '',
   table_default_page_size: tablePageSizeDefault,
   table_page_size_options: [10, 20, 50, 100],
   custom_menu_items: [] as Array<{id: string; label: string; icon_svg: string; url: string; visibility: 'user' | 'admin'; sort_order: number}>,
@@ -3077,8 +3075,6 @@ async function saveSettings() {
       home_content: form.home_content,
       backend_mode_enabled: form.backend_mode_enabled,
       hide_ccs_import_button: form.hide_ccs_import_button,
-      purchase_subscription_enabled: form.purchase_subscription_enabled,
-      purchase_subscription_url: form.purchase_subscription_url,
       table_default_page_size: form.table_default_page_size,
       table_page_size_options: form.table_page_size_options,
       custom_menu_items: form.custom_menu_items,