feat: i18n with en + zh-CN, namespaced keys, system locale auto-detect

[hqhq1025]

Summary

Ships internationalization infrastructure for the renderer, with English and Simplified Chinese as the tier-1 locales. Adds the @open-codesign/i18n workspace package, per-locale demo prompts in @open-codesign/templates, an Electron locale IPC channel, and the renderer wiring needed to swap the entire UI live.

Namespaces

All keys live in packages/i18n/src/locales/{en,zh-CN}.json. Top-level namespaces:

• common — appName, send, cancel, retry, save, close, settings, advanced, about, learnMore, copy, copied, comingSoon, loading, preAlpha, tagline
• preview — empty.{title,body,starterChip}, loading.title, error.{title,body,copyError}, ready, noDesign
• chat — placeholder, sendShortcut, emptyHint
• settings — title, tabs.{models,appearance,storage,advanced}, language.{label,system}, theme.{label,light,dark,system}
• onboarding — welcome.{title,subtitle,tryFree,useKey,useOllama}, paste.{title,placeholder,recognized,connected_one,connected_other,howToGet,errors.{401,402,429,network}}, choose.{title,primary,fast,estimatedCost,start}
• commands — title, placeholder, items.{newDesign,toggleTheme,openSettings,export}
• errors — generic, providerAuthMissing, providerError, ipcBadInput, exporterNotReady
• demos — meditationApp, caseStudy, pitchDeck, marketingLanding (each with title/description/prompt)

Dependencies added

Package	Size	License
i18next@^23.16.5	~50 KB	MIT
react-i18next@^15.1.3	~25 KB	MIT

No other production deps. Stays well under the 30-prod-dep / 80 MB install budget.

Missing-key policy (PRINCIPLES §10)

Never silent. Concretely:

• missingKeyHandler logs console.warn with key path + namespace + active locale.
• parseMissingKeyHandler renders ⟦key⟧ in dev mode (loud) and the raw key in prod (visible, never a fake fallback).
• normalizeLocale warns when given an unsupported locale and falls back to en.

Locale persistence

Saved to ~/.config/open-codesign/locale.json via the new locale:get-system / locale:get-current / locale:set IPC channels. This lives outside config.toml until wt/onboarding lands; once it does, fold the value into [ui] locale = "zh-CN" and remove the standalone file.

Integration notes for parallel worktrees

wt/onboarding — onboarding namespace is reserved with the keys you need:

• onboarding.welcome.{title,subtitle,tryFree,useKey,useOllama}
• onboarding.paste.{title,placeholder,recognized,connected_one,connected_other,howToGet,errors.{401,402,429,network}}
• onboarding.choose.{title,primary,fast,estimatedCost,start}

Use useT() from @open-codesign/i18n. Pluralization works via the _one / _other suffix; interpolation via {{name}}. When you wire config.toml, read ui.locale and call useCodesignStore.getState().setLocale(value) on boot, then delete apps/desktop/src/main/locale-ipc.ts's file persistence (keep the IPC channel, switch the body to read the TOML).

wt/preview-ux — keys provided in the settings, preview, commands namespaces. Settings → Appearance should render a language picker bound to setLocale() from the store (showing availableLocales from @open-codesign/i18n). Top bar tagline is common.tagline, pre-alpha pill is common.preAlpha.

Test plan

• pnpm install — clean
• pnpm -r typecheck — green
• pnpm lint — green (1 pre-existing warning unchanged)
• pnpm -r test — green (6 new vitest cases in @open-codesign/i18n)
• pnpm --filter @open-codesign/desktop dev and verify:
  • app boots in system locale (zh-CN on Chinese macOS, en otherwise)
  • starter prompts render translated titles + descriptions
  • chat placeholder reads "想设计什么？" in zh-CN, "Describe what to design…" in en
  • pre-alpha tag reads "预览版" in zh-CN
  • useCodesignStore.getState().setLocale('zh-CN') from DevTools swaps the UI live
  • unsupported locale (setLocale('fr')) falls back to en with a console warning
  • kill + relaunch restores the previous locale from ~/.config/open-codesign/locale.json

Files NOT touched

Per worktree boundaries: apps/desktop/src/renderer/src/onboarding/**, apps/desktop/src/renderer/src/components/**, apps/desktop/src/main/{config,keychain,onboarding-ipc}.ts, and packages/{providers,runtime,exporters,artifacts,core,ui,shared}/**.

Signed-off-by: dco

[hqhq1025]

Superseded by v2 (clean cherry-pick on top of latest main). See 2-v2 PR.