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

server/utils/db-init.ts

@@ -272,6 +272,7 @@ async function initializeDatabase() {
       event_id text references dinner_events(id) on delete restrict,
       customer_name text not null,
       customer_phone text not null,
+      locale text not null default 'en',
       booking_mode_id text references booking_modes(id) on delete restrict,
       booking_mode text not null,
       quantity integer not null check (quantity >= 1),
@@ -316,6 +317,11 @@ async function initializeDatabase() {
     add column if not exists remark text
   `
 
+  await sql`
+    alter table bookings
+    add column if not exists locale text not null default 'en'
+  `
+
   await sql`
     create unique index if not exists bookings_receipt_token_idx
     on bookings (receipt_token)