feat(i18n): add full-stack internationalization support

Add languages and entity_translations tables to database schema
Implement localized queries and translation management in backend
Integrate frontend i18n and add translation UI components

backend/db/schema.sql

@@ -3,6 +3,52 @@ CREATE TABLE IF NOT EXISTS environments (
   name text NOT NULL UNIQUE
 );
 
+CREATE TABLE IF NOT EXISTS languages (
+  code text PRIMARY KEY,
+  name text NOT NULL,
+  enabled boolean NOT NULL DEFAULT true,
+  is_default boolean NOT NULL DEFAULT false,
+  sort_order integer NOT NULL DEFAULT 0 CHECK (sort_order >= 0),
+  CHECK (code ~ '^[a-z]{2}(-[A-Z]{2})?$'),
+  CHECK (length(name) BETWEEN 1 AND 80)
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS languages_single_default_idx
+  ON languages (is_default)
+  WHERE is_default = true;
+
+INSERT INTO languages (code, name, enabled, is_default, sort_order)
+VALUES
+  ('en', 'English', true, true, 10),
+  ('zh-CN', '简体中文', true, false, 20)
+ON CONFLICT (code) DO NOTHING;
+
+CREATE TABLE IF NOT EXISTS entity_translations (
+  entity_type text NOT NULL CHECK (
+    entity_type IN (
+      'pokemon',
+      'skills',
+      'environments',
+      'favorite-things',
+      'item-categories',
+      'item-usages',
+      'acquisition-methods',
+      'items',
+      'maps',
+      'habitats',
+      'daily-checklist-items'
+    )
+  ),
+  entity_id integer NOT NULL,
+  locale text NOT NULL REFERENCES languages(code) ON DELETE CASCADE,
+  field_name text NOT NULL CHECK (field_name IN ('name', 'title')),
+  value text NOT NULL,
+  PRIMARY KEY (entity_type, entity_id, locale, field_name)
+);
+
+CREATE INDEX IF NOT EXISTS entity_translations_lookup_idx
+  ON entity_translations (entity_type, entity_id, field_name, locale);
+
 CREATE TABLE IF NOT EXISTS users (
   id integer GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
   email text NOT NULL UNIQUE,