chore: set Japanese as default locale and remove i18n-cli

- Remove @lobehub/i18n-cli package and .i18nrc.js config
- Delete locales/default/ directory (translation source)
- Change DEFAULT_LOCALE and fallbackLng to ja-JP
- Reorder LOCALES array with Japanese first
- Simplify language selector to horizontal solid button style

package.json

@@ -17,8 +17,7 @@
     "format": "prettier --write \"src/**/*.{js,jsx,ts,tsx,css,json}\"",
     "test": "vitest run",
     "test:watch": "vitest",
-    "test:coverage": "vitest run --coverage",
-    "i18n": "lobe-i18n"
+    "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
     "@codemirror/lang-css": "^6.3.0",
@@ -46,7 +45,6 @@
   },
   "devDependencies": {
     "@electron-toolkit/tsconfig": "^1.0.1",
-    "@lobehub/i18n-cli": "^1.26.1",
     "@types/node": "^22.0.0",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",