Add Japanese locale (ja) support (#1810)

* Add Japanese locale (ja) support

- New: ja.ts with 2624 UI translation keys
- New: ja.settings.ts with 1789 settings translation keys
- Registered ja in runtime.ts (Locale type, LOCALES, LOCALE_LABEL_KEYS, normalizeLocale)
- Added ja-JP BCP-47 mapping in intl.ts
- Added JA_MESSAGES in bootstrap.ts
- Added ja to lazy-load dictionary map in store.ts
- Updated messages.test.ts with ja dictionary
- Added common.language.japanese to all 8 existing locale files

* fix(i18n): complete Japanese locale updates

* test(i18n): enforce locale key parity

* docs: add Japanese documentation

---------

Co-authored-by: yuchi0531 <yuchi0531@users.noreply.github.com>
Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/lib/i18n/messages.test.ts

@@ -3,6 +3,7 @@ import { describe, expect, test } from 'bun:test';
 import { dict as enDict } from './messages/en';
 import { dict as esDict } from './messages/es';
 import { dict as frDict } from './messages/fr';
+import { dict as jaDict } from './messages/ja';
 import { dict as koDict } from './messages/ko';
 import { dict as plDict } from './messages/pl';
 import { dict as ptBrDict } from './messages/pt-BR';
@@ -14,6 +15,7 @@ const localeDictionaries = {
   en: enDict,
   fr: frDict,
   es: esDict,
+  ja: jaDict,
   'pt-BR': ptBrDict,
   uk: ukDict,
   ko: koDict,
@@ -23,15 +25,18 @@ const localeDictionaries = {
 } as const;
 
 describe('i18n dictionaries', () => {
-  test('french stays in key parity with english', () => {
+  test('all locales stay in key parity with english', () => {
     const englishKeys = Object.keys(enDict).sort();
 
-    expect(Object.keys(frDict).sort()).toEqual(englishKeys);
+    for (const dictionary of Object.values(localeDictionaries)) {
+      expect(Object.keys(dictionary).sort()).toEqual(englishKeys);
+    }
   });
 
-  test('all locales expose the french language label key', () => {
+  test('all locales expose language label keys', () => {
     for (const [, dictionary] of Object.entries(localeDictionaries)) {
       expect(dictionary['common.language.french']).toBeTruthy();
+      expect(dictionary['common.language.japanese']).toBeTruthy();
     }
   });
 });