feat(i18n): add multi-language support (en/zh) across app and server

Implement useLocale composable and shared translation dictionaries
Translate public pages, booking flow, and receipt views
Store booking locale to send localized WhatsApp notifications

shared/i18n.ts

@@ -0,0 +1,43 @@
+export const SUPPORTED_LOCALES = ['en', 'zh'] as const
+export type AppLocale = typeof SUPPORTED_LOCALES[number]
+
+export const DEFAULT_LOCALE: AppLocale = 'en'
+
+export const LOCALE_OPTIONS: Array<{
+  value: AppLocale
+  label: string
+  shortLabel: string
+}> = [
+  {
+    value: 'en',
+    label: 'English',
+    shortLabel: 'EN'
+  },
+  {
+    value: 'zh',
+    label: '中文',
+    shortLabel: '中'
+  }
+]
+
+export function resolveLocale(value: string | null | undefined, fallback: AppLocale = DEFAULT_LOCALE): AppLocale {
+  const normalized = String(value || '').trim().toLowerCase()
+
+  if (!normalized) {
+    return fallback
+  }
+
+  if (normalized === 'zh' || normalized.startsWith('zh-') || normalized.startsWith('cn')) {
+    return 'zh'
+  }
+
+  if (normalized === 'en' || normalized.startsWith('en-')) {
+    return 'en'
+  }
+
+  return fallback
+}
+
+export function getOppositeLocale(locale: AppLocale): AppLocale {
+  return locale === 'zh' ? 'en' : 'zh'
+}