feat(webui): redesign frontend with cute theme, i18n and theme switching

- Redesign UI with neko-themed kawaii style (cat ears logo, cat face SVG,
  soft pastel pink/purple palette, rounded corners, soft shadows)
- Add light/dark theme switching (Cherry Blossom / Night Sakura) with
  localStorage persistence
- Add i18n support for English, Simplified Chinese, and Japanese with
  translations loaded from separate JSON files
- Separate monolithic index.html into index.html, style.css, and app.js
- Improve chat UI with directional message bubbles (user right, bot left)
- Add responsive layout for mobile devices
- Add Enter key submit on login/init forms
- Preserve all existing functionality (auth, chat, providers, channels,
  config, status)

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

Author: czyt