Internationalization (i18n): support English and Japanese

[nyatinte]

AS IS

All user-facing strings are hardcoded in English throughout the codebase:

• UI prompts: "Select package", "Select script" (src/ui.ts)
• Cancellation message: "Cancelled." (src/ui.ts)
• Error messages: "No packages match: ...", "Multiple packages match: ...", "No scripts in ..." (src/index.ts)
• Workspace error: "Run prw from workspace root." (src/workspace.ts)

Japanese-speaking users see English-only output with no way to change it.

TO BE

Detect locale from the environment (via `LANG` / `LC_ALL` env vars or `Intl.DateTimeFormat().resolvedOptions().locale`) and display messages in the matching language. Fall back to English if the locale is not supported.

Supported locales in v1:

• en (English) — default/fallback
• ja (Japanese)

Example (Japanese locale):

```
パッケージを選択:
❯ @myapp/web apps/web
@myapp/api apps/api

スクリプトを選択:
❯ dev next dev
build tsc && next build
```

Implementation notes:

• Introduce a src/i18n.ts module exporting a typed message catalog and a `t(key)` helper
• All hardcoded strings in `ui.ts`, `index.ts`, and `workspace.ts` replaced with `t()` calls
• The active locale is resolved once at startup and passed down (or stored as a module-level singleton)
• No runtime dependencies required — plain object lookup is sufficient