feat(i18n): add machine-drafted translations and contributor docs (U8)

Fill zh-CN, zh-TW, fr, and es catalogs for the migrated keys. zh-CN and zh-TW
are independently localized (Simplified vs Traditional script + Taiwan
vocabulary, e.g. 项目/任务 vs 專案/工作), not script-converted. Translations are
machine-drafted and flagged for human review in locales/TRANSLATION_STATUS.md.

Add docs/i18n-contributing.md (translate / add-a-language / --lang usage) and
cross-link it from contributing.md and cli-reference.md. i18n:status reports
100% for all four locales across the migrated key set.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/cli-reference.md

@@ -207,8 +207,14 @@ fn dashboard --no-auth                     # disable bearer auth (local only)
 fn dashboard --interactive
 fn dashboard --paused
 fn dashboard --dev
+fn dashboard --lang zh-TW                   # force a UI locale for this run
 ```
 
+The terminal UI is localized. `--lang <code>` (one of `en`, `zh-CN`, `zh-TW`,
+`fr`, `es`) takes precedence over the saved dashboard language setting and the
+`LC_ALL`/`LC_MESSAGES`/`LANG`/`LANGUAGE` environment. See
+[Localization contributor guide](./i18n-contributing.md).
+
 | Option | Description |
 |---|---|
 | `--port`, `-p` | Dashboard HTTP port (default `4040`). |