feat: add Japanese (ja) localization

Add full Japanese translation for the desktop app UI, register the ja
locale in i18n config, and add the language option to the preferences
selector.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: muraki-sports

apps/desktop/src/i18n/locales/en.json

@@ -395,7 +395,8 @@
         "description": "Choose the language used throughout the app",
         "options": {
           "system": "System",
-          "en": "English"
+          "en": "English",
+          "ja": "日本語"
         },
         "restartDialog": {
           "title": "Restart required",