feat(i18n): add Traditional Chinese (zh-TW) locale support

[Celina-create]

Summary

Add full Traditional Chinese (繁體中文) UI support to the Nexu client, as requested in #1120.

• New locale file: apps/web/src/i18n/locales/zh-TW.ts — all 1190+ translation keys converted from Simplified Chinese via OpenCC s2twp (Taiwan-standard phrases)
• i18next registration: zh-TW resource added alongside existing en and zh
• Locale detection: Browser navigator.language of zh-TW or zh-HK auto-selects Traditional Chinese
• Language switcher: Both the Welcome page dropdown and Settings page selector now show three options: English / 简体中文 / 繁體中文
• Desktop renderer: resolveLocale() helper updated with optional zh-TW key support and fallback to zh
• Desktop sync: syncDesktopLocale() and bootstrapLocale() properly handle zh-TW round-trip

Changed files

File	Change
apps/web/src/i18n/locales/zh-TW.ts	New — full Traditional Chinese translation
apps/web/src/i18n/index.ts	Register zh-TW resource, update detection
apps/web/src/hooks/use-locale.tsx	Extend Locale type, detection, sync, bootstrap
apps/web/src/components/language-switcher.tsx	Add 繁體中文 option
apps/web/src/pages/models.tsx	Add 繁體中文 to settings language selector
apps/desktop/src/lib/i18n.ts	Support zh-TW in desktop renderer helper

Closes #1120

Test plan

• Switch language to 繁體中文 in Welcome page language switcher — UI strings display in Traditional Chinese
• Switch language to 繁體中文 in Settings → General → Language — UI strings update correctly
• Verify browser with zh-TW locale auto-detects Traditional Chinese on first load
• Verify browser with zh-HK locale also auto-detects Traditional Chinese
• Switch between English → 简体中文 → 繁體中文 and back — no rendering issues
• Refresh page after selecting 繁體中文 — locale persists via localStorage
• Verify desktop app renderer picks up zh-TW when available

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[chatgpt-codex-connector]

Align zh-TW locale sync with desktop API contract

When a user picks Traditional Chinese, syncDesktopLocale() now sends "zh-TW" to /api/internal/desktop/preferences, but the controller route still validates locale as only "en" | "zh-CN" (apps/controller/src/routes/desktop-routes.ts, checked desktopPreferencesUpdateSchema). That request is rejected (400) and then swallowed by .catch, so the UI appears to switch but desktop preferences never persist server-side and controller/runtime features that rely on stored locale remain out of sync.

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[sentry]

Codecov Report

❌ Patch coverage is 0.08285% with 1206 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
apps/web/src/i18n/locales/zh-TW.ts	0.00%	1152 Missing ⚠️
apps/web/src/hooks/use-locale.tsx	0.00%	28 Missing ⚠️
apps/web/src/pages/models.tsx	0.00%	12 Missing ⚠️
apps/desktop/src/lib/i18n.ts	14.28%	6 Missing ⚠️
apps/web/src/i18n/index.ts	0.00%	6 Missing ⚠️
apps/web/src/components/language-switcher.tsx	0.00%	2 Missing ⚠️

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