feat: rebrand to ConvertX-CN, fix TypeScript error, add docs

- Fix TypeScript TS2345 error in i18n/service.ts by parsing cookie from header
- Rebrand all ConvertX references to ConvertX-CN
- Change default language from en to zh-TW
- Fix footer to always display English 'Powered by ConvertX-CN'
- Update GitHub links to pi-docket/ConvertX-CN
- Add /docs folder with documentation:
  - getting-started.md
  - configuration.md
  - converters.md
  - i18n.md
- Update all locale files with new branding

src/locales/ja.json

@@ -1,6 +1,6 @@
 {
   "common": {
-    "appName": "ConvertX",
+    "appName": "ConvertX-CN",
     "poweredBy": "Powered by",
     "version": "v{version}",
     "loading": "読み込み中...",
@@ -84,7 +84,7 @@
     "noCookies": "認証Cookieが見つかりません。ブラウザがCookieをブロックしている可能性があります。"
   },
   "setup": {
-    "welcome": "ConvertXへようこそ！",
+    "welcome": "ConvertX-CNへようこそ！",
     "createYourAccount": "アカウントを作成",
     "reportIssues": "問題があれば報告",
     "github": "GitHub"