feat: i18n infrastructure + en/zh-CN translations + per-locale demos

[hqhq1025]

Goal

Land the i18n substrate so the app can ship in English and Simplified Chinese without any further infrastructure work. Strings, demos, and the main-process locale IPC are all in place. Renderer wiring (App.tsx, store, preload) is intentionally not in this PR — those files are owned by the parallel wt/preview-ux-v2 branch and will be hooked up by the maintainer in a follow-up integration commit.

This is the rebuilt successor to the abandoned wt/i18n branch: same substance, clean diff against today's main, zero touches to renderer files.

Files in

Path	Purpose
packages/i18n/package.json + tsconfig.json	New workspace package @open-codesign/i18n
packages/i18n/src/index.ts	initI18n / setLocale / useT / normalizeLocale / isSupportedLocale / availableLocales / getCurrentLocale
packages/i18n/src/locales/{en,zh-CN}.json	Full key tree across common, preview, chat, settings, onboarding, commands, errors, demos
packages/i18n/src/i18n.test.ts	8 vitest cases: normalization, support detection, live switching, missing-key warning
packages/templates/src/locales/{en,zh-CN}.ts	Per-locale DemoTemplate[] for the four built-in demos
packages/templates/src/index.ts	Adds getDemos(locale) / getDemo(id, locale). BUILTIN_DEMOS retained as English alias for back-compat.
packages/templates/package.json	Adds @open-codesign/i18n workspace dep
apps/desktop/src/main/locale-ipc.ts	registerLocaleIpc() exposing locale:get-system, locale:get-current, locale:set — persists to ~/.config/open-codesign/locale.json (separate from config.toml, with schemaVersion: 1)
docs/I18N.md	How to add a string, add a locale, naming convention

Files out (parallel-worktree boundary — NOT touched)

• apps/desktop/src/main/index.ts
• apps/desktop/src/preload/index.ts
• apps/desktop/src/renderer/src/App.tsx
• apps/desktop/src/renderer/src/store.ts
• apps/desktop/src/renderer/src/main.tsx
• apps/desktop/src/renderer/src/onboarding/**
• apps/desktop/src/renderer/src/components/**

New production dependencies

Dep	Version	Size (min+gzip)	License	Why
i18next	^23.16.5	~14 KB gzip (~50 KB raw)	MIT	De-facto JS i18n core; tree-shakable, no runtime fetching
react-i18next	^15.1.3	~7 KB gzip (~25 KB raw)	MIT	React bindings (useTranslation hook) for i18next

Both are MIT, Apache-2.0 compatible, zero transitive bloat. Combined raw cost: < 80 KB. Well inside the §1 lean budget. No alternatives considered seriously: react-intl is ~3x the size, hand-rolling means re-implementing pluralization rules.

Hard rules respected

• No silent fallbacks (§10): missing keys log console.warn(key, namespace, locale) and render as \u27E6key\u27E7 in dev so they jump out in the UI.
• Schema-versioned persistence (§5b): locale file carries schemaVersion: 1.
• TypeScript strict, no any, bracket notation, import type for type-only imports.
• Lazy-friendly: i18next initializes on first render via initI18n(locale); resources are inlined JSON, no async fetch.
• DCO sign-off on every commit.

Integration notes for the maintainer

After wt/preview-ux-v2 lands, wire up the renderer in a single follow-up commit. Three steps:

1. Main process — apps/desktop/src/main/index.ts

import { registerLocaleIpc } from './locale-ipc';
// inside app.whenReady():
registerLocaleIpc();

2. Preload — apps/desktop/src/preload/index.ts

Add to the bridged API:

locale: {
  getSystem: () => ipcRenderer.invoke('locale:get-system'),
  getCurrent: () => ipcRenderer.invoke('locale:get-current'),
  set: (locale: string) => ipcRenderer.invoke('locale:set', locale),
}

3. Renderer — apps/desktop/src/renderer/src/main.tsx

import { initI18n } from '@open-codesign/i18n';
const initialLocale = await window.electronAPI.locale.getCurrent();
await initI18n(initialLocale);
// then mount <App />

4. t(...) migration table

Replace each hard-coded string in App.tsx, onboarding, and components with the matching key. Use const t = useT() at the top of each component.

Hard-coded string	New call
'pre-alpha' chip	t('common.preAlpha')
'BYOK · local-first · multi-model'	t('common.tagline')
'Send' button	t('common.send')
Settings labels	t('settings.title'), t('settings.tabs.models'), etc.
Welcome screen title	t('onboarding.welcome.title')
Welcome subtitle	t('onboarding.welcome.subtitle')
Try-free / use-key / use-Ollama buttons	t('onboarding.welcome.{tryFree,useKey,useOllama}')
Paste-key title / placeholder	t('onboarding.paste.{title,placeholder}')
Detected provider toast	t('onboarding.paste.recognized', { provider })
Connected models count	t('onboarding.paste.connected', { count })
Advanced base-URL section	t('onboarding.paste.advanced.{title,description}')
Paste-key error 401/402/429/network	t('onboarding.paste.errors.{401,402,429,network}')
Choose model title / labels	t('onboarding.choose.{title,primary,fast,start}'), t('onboarding.choose.estimatedCost', { amount })
Preview empty state	t('preview.empty.{title,body,starterChip}')
Preview loading	t('preview.loading.title')
Preview error	t('preview.error.{title,body,copyError}')
Chat input placeholder	t('chat.placeholder')
Chat send shortcut hint	t('chat.sendShortcut')
Command palette items	t('commands.{title,placeholder}'), t('commands.items.{newDesign,toggleTheme,openSettings,export}')
Generic error toast	t('errors.generic') / t('errors.providerError', { message }) / t('errors.providerAuthMissing')

For demos, swap BUILTIN_DEMOS import to getDemos(currentLocale) (and have the store hold currentLocale). The BUILTIN_DEMOS alias stays in place during migration so nothing breaks mid-flight.

5. Settings → Language dropdown

Three options: System default (passes undefined so normalizeLocale falls back to app.getLocale()), English, 中文. On change: await window.electronAPI.locale.set(value); await setLocale(value); and re-render.

§5b checklist

• Compatible: ✅ — BUILTIN_DEMOS alias kept; new IPC channels are net-additive; locale file is schema-versioned.
• Upgradeable: ✅ — adding a third locale is an add-only change (one JSON, one TS file, one availableLocales entry). Documented in docs/I18N.md.
• No bloat: ✅ — two prod deps totaling < 80 KB gzipped; both peer-friendly; no transitive heavies pulled in.
• Elegant: ✅ — no silent fallbacks, missing keys visibly fail in dev, idiomatic Chinese (not machine-literal), demo prompts feel native in both languages.

Verification

pnpm install          # ok
pnpm -r typecheck     # green across all 11 packages
pnpm lint             # green (1 pre-existing warning unrelated to this PR)
pnpm -r test          # 28 tests pass (8 new in i18n)