refactor(ui): Replace axios with fetch in all pages and extract auth layout
- Replace all $axios calls with native fetch across 8 page files
- Add inline send-code button to login page code flow
- Extract animated bubble background into shared auth layout
- Restructure langs.json with zh-CN first, add new i18n keys
- Add scoped langs.json for auth.send_code translation
- Wire auth layout to login and callback routes
- Fix mobile sign-up panel transform direction
- Use onAuthSuccess for login/register code flows
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