feat(i18n): add Simplified Chinese (zh-CN) localization (#219) (#239)

Closes #219.

## What

Adds Simplified Chinese alongside existing English and Traditional
Chinese. The reporter assumed React, but the app is Svelte and already
has a working i18n framework with `zh-TW` — this just adds the third
locale.

## How

- New `locales/zh-CN.ts` with all 721 strings. Initial pass is
  a mechanical Traditional → Simplified character conversion with
  CN-specific vocabulary swaps applied: 项目 (vs 專案), 设置 (vs 設定),
  文件 (vs 檔案), 加载 (vs 載入), 刷新 (vs 重新整理), 默认 (vs 預設),
  软件 (vs 軟體), 视频 (vs 影片), 仓库 (vs 儲存庫), 服务器 (vs 伺服器),
  消息 (vs 訊息), 命令 (vs 指令 for slash commands kept as 命令 to
  match CN convention), 默认 (vs 預設), 重启 (vs 重新啟動), etc.

- Locale switcher labels updated from `EN` / `中文` (ambiguous) to
  `EN` / `简` / `繁` so users can distinguish the two Chinese options
  at a glance. `zh-CN` listed before `zh-TW` since CN has the larger
  user base; cycle order is EN → 简 → 繁 → EN.

- `Locale` type union extended with `'zh-CN'`. The existing detect
  logic already handles bare `zh` browser preferences via prefix
  matching — `zh-CN` browsers will now match directly instead of
  falling through to `zh-TW`.

- TypeScript validates 1:1 key coverage via
  `type Translations = Record<TranslationKey, string>` where
  `TranslationKey = keyof typeof en`. `npm run check` confirms zero
  missing keys (still at 11-error main baseline, no new errors).

## Native-speaker review wanted

This is a translator-as-second-language pass. Some phrasing may feel
stiff or use slightly off vocabulary for the context. Inviting
follow-up PRs from native zh-CN speakers (especially the issue
reporter) — file is structured in section comments mirroring the
en/zh-TW files so spot-fixes are easy.

## Test plan

- [x] `npm run check` — 11 errors (same as main; Translations type
      validated all 721 keys present)
- [x] `npx vitest run` — 1567/1567 pass
- [ ] Manual: launch app on a `zh-CN` system, confirm UI renders
      Simplified Chinese, test language switcher cycles EN → 简 → 繁

Co-authored-by: Tyler Gray <tylerg@emergentsoftware.net>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

src/lib/i18n/i18n.svelte.ts

@@ -1,15 +1,18 @@
 import { en } from './locales/en';
 import { zhTW } from './locales/zh-TW';
+import { zhCN } from './locales/zh-CN';
 import type { Locale, LocaleInfo, TranslationKey, Translations } from './types';
 
 const locales: Record<Locale, Translations> = {
 	en,
-	'zh-TW': zhTW
+	'zh-TW': zhTW,
+	'zh-CN': zhCN
 };
 
 const localeInfos: LocaleInfo[] = [
 	{ code: 'en', label: 'EN' },
-	{ code: 'zh-TW', label: '中文' }
+	{ code: 'zh-CN', label: '简' },
+	{ code: 'zh-TW', label: '繁' }
 ];
 
 function detectLocale(): Locale {