feat(admin): let admins set a default currency for new users

Adds a currency picker to Admin > User Defaults. Stored as the default_currency
user-default, so users who have not picked their own currency inherit it in
Costs.

client/src/components/Admin/DefaultUserSettingsTab.tsx

@@ -6,6 +6,7 @@ import { useToast } from '../shared/Toast'
 import Section from '../Settings/Section'
 import CustomSelect from '../shared/CustomSelect'
 import { MapView } from '../Map/MapView'
+import { CURRENCIES, SYMBOLS } from '../Budget/BudgetPanel.constants'
 import type { Place } from '../../types'
 
 const MAP_PRESETS = [
@@ -20,6 +21,7 @@ type Defaults = {
   temperature_unit?: string
   dark_mode?: string | boolean
   time_format?: string
+  default_currency?: string
   blur_booking_codes?: boolean
   map_tile_url?: string
   map_provider?: string
@@ -226,6 +228,23 @@ export default function DefaultUserSettingsTab(): React.ReactElement {
         ))}
       </OptionRow>
 
+      {/* Default Currency */}
+      <div>
+        <label className="block text-sm font-medium mb-1.5 text-content-secondary">
+          {t('settings.currency')} <ResetButton field="default_currency" />
+        </label>
+        <CustomSelect
+          value={defaults.default_currency || ''}
+          onChange={(value: string) => { if (value) save({ default_currency: value }) }}
+          placeholder={t('settings.currency')}
+          searchable
+          options={CURRENCIES.map(c => ({ value: c, label: SYMBOLS[c] ? `${c}  ${SYMBOLS[c]}` : c }))}
+          size="sm"
+          style={{ maxWidth: 240 }}
+        />
+        <p className="text-xs mt-1 text-content-faint">{t('settings.currencyHint')}</p>
+      </div>
+
       {/* Blur Booking Codes */}
       <OptionRow label={<>{t('settings.blurBookingCodes')} <ResetButton field="blur_booking_codes" /></>}>
         {([

server/src/services/settingsService.ts

@@ -10,6 +10,9 @@ export const DEFAULTABLE_USER_SETTING_KEYS = [
   'temperature_unit',
   'dark_mode',
   'time_format',
+  // Instance-wide default currency for Costs (new users inherit it until they
+  // pick their own). Free-form ISO code, validated on the client.
+  'default_currency',
   'blur_booking_codes',
   'map_tile_url',
   // Instance-wide Mapbox defaults: an admin can set a shared token + style so the