feat: add i18n support with Simplified Chinese (zh-CN) translation

[yiliqi78]

Summary

• Add internationalization (i18n) infrastructure using react-i18next with browser language auto-detection
• Include complete Simplified Chinese (zh-CN) translation covering ~271 keys across 17 modules
• Add LanguageSwitcher component (globe icon in titlebar) for easy language switching
• Convert 4 key components as working examples: CustomTitlebar, ProjectList, Topbar, Settings

Motivation

As noted in the README's contributing section, internationalization is listed as an area for contribution. This PR establishes the i18n foundation and provides a complete Chinese translation, making opcode accessible to Chinese-speaking developers.

What's included

Infrastructure

• src/i18n/index.ts — i18next config with LanguageDetector (auto-detects browser locale, persists choice to localStorage)
• src/i18n/locales/en.json — English translation keys extracted from source
• src/i18n/locales/zh-CN.json — Simplified Chinese translations
• src/components/LanguageSwitcher.tsx — Dropdown component for switching languages

Converted components (as examples)

Component	Scope
CustomTitlebar.tsx	Tooltip labels, dropdown menu items
ProjectList.tsx	All user-facing strings (title, buttons, empty state)
Topbar.tsx	Status indicator, version display, install prompts
Settings.tsx	Header, 8 tab labels, all form labels/descriptions, toast messages, permission rules

Translation conventions

English	Chinese	Rationale
Agent	智能体	Standard AI term in Chinese (not 代理)
Checkpoint	检查点
Token	Token	Industry standard, kept as-is
Claude Code / opcode	Claude Code / opcode	Brand names, kept as-is
Hook	钩子
MCP	MCP (模型上下文协议)	Abbreviation + full name

How to convert remaining components

The pattern is straightforward — each component just needs:

import { useTranslation } from "react-i18next";

// Inside the component:
const { t } = useTranslation();

// Replace hardcoded text:
// Before: <h1>Projects</h1>
// After:  <h1>{t("projects.title")}</h1>

// With interpolation:
// Before: <span>View all ({count})</span>
// After:  <span>{t("projects.viewAll", { count })}</span>

The remaining ~50 components can be converted incrementally in follow-up PRs.

Dependencies added

• i18next — Core i18n framework
• react-i18next — React bindings
• i18next-browser-languagedetector — Auto-detect user's browser language

Test plan

• App starts without errors with default English locale
• Switching to zh-CN via globe icon in titlebar changes all converted component text to Chinese
• Language preference persists across app restarts (stored in localStorage)
• Settings page tab labels, form labels, and toast messages display correctly in both languages
• ProjectList empty state, headers, and buttons display correctly in both languages
• Topbar version status and install prompts display correctly in both languages
• TypeScript compilation passes (tsc --noEmit)