feat: add lazy-loaded i18n with multi language support

[SCPZ24]

feat: add lazy-loaded i18n with English / Japanese / Chinese support

Summary

This PR introduces multi-language UI support to Clypra. Since the entire UI
is a single React app shared by the Tauri desktop shell and the Capacitor
iOS/Android shells (all loading the same dist/ bundle), the localization is
implemented purely in the React layer — no native Swift/Kotlin/Gradle changes
are required, and all three platforms get translations automatically.

Users can switch languages via Settings → Appearance → Language
(English / 日本語 / 简体中文). The selection is persisted and restored on
next launch.

Motivation

The UI strings were previously hardcoded in English, making the editor
inaccessible to non-English speakers. This adds a proper i18n foundation so
the community can grow translation coverage incrementally.

Implementation

• Framework: react-i18next + i18next.
• Lazy loading: each locale lives in its own JSON
  (src/i18n/locales/{en,ja,zh-CN}.json) and is pulled in via dynamic
  import(), so Vite emits a separate chunk per language. Only the active
  language plus the en fallback are loaded at startup; other languages are
  fetched on demand the first time the user selects them, then cached.
• State: a persisted language field added to the existing Zustand
  settingsStore, mirroring the existing theme/font pattern. setLanguage
  updates the UI state immediately, then awaits the chunk load before switching.
• Startup: main.tsx awaits initI18n(persistedLanguage) before the first
  render (with first-run navigator.language detection) to avoid a key-flash.
• CJK fonts: when switching to Japanese/Chinese, CJK fallback fonts
  (PingFang SC, Hiragino Sans, Microsoft YaHei, Noto Sans CJK SC) are appended
  to --font-sans to prevent missing glyphs.
• String extraction: migrated hardcoded strings to t() across the
  Settings modal, TopBar, all media tabs, the properties panel, the Launch
  screen, and the Export dialog, with natural Japanese and Simplified Chinese
  translations.

Scope / follow-ups

Intentionally left for a later pass: timeline components, preview/transport
controls, cache settings, effect/template cards, context menus, and
mobile/viewport chrome. These are mostly covered by tests asserting English
text; migrating them cleanly first requires the test setup to initialize i18n.
This is a non-breaking, additive change (default language remains English).

Testing

• npm run build passes tsc type-checking cleanly.
• Verified dist/assets/ contains separate lazy chunks for en, ja, and
  zh-CN.
• Manually switched between all three languages: instant after first load,
  persists across reload, and CJK text renders without missing glyphs.

Manual testing checklist:

• No console errors
• Language switch works on desktop (Tauri)
• Verified on iOS/Android (Capacitor) — needs a reviewer with mobile setup
• Tested on Windows / Linux