Internationalization: 7 languages with live switching

Custom signal-based runtime (no i18n dependency): shared src/shared/i18n
dictionaries, ui I18nService.t() reactive by construction so language
switches live, and a backend i18n.ts for native menus + thrown errors.
Every user-facing string keyed across templates, toasts, dialogs, file
kinds, columns, macOS menu/context menus, and errors. Language picker in
Options (auto-detects OS locale, falls back to English); switching
relabels the webview and native chrome instantly. English/Spanish/French/
German fully drafted; Chinese/Japanese/Basque drafted, flagged for native
review. Docs updated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/gotchas.md

@@ -85,6 +85,25 @@ touching the adjacent code.
   don't shrink the 400 ms debounce.
 - `panel.touched` gates localDir auto-navigation; `connectToken` guards
   against a slow connect resolving into a panel the user already reused.
+
+## i18n
+
+- **Translations live in `src/shared/i18n/*.ts` (TS, not JSON)** — deliberate:
+  it sidesteps `resolveJsonModule` differences between the Angular esbuild and
+  Bun builds and gives full typing. `en.ts` is the source of truth; other
+  locales are partial and fall back to English then the raw key.
+- **`i18n.t()` is reactive because it reads the `locale` signal** — that's what
+  makes live language switching work with no pipe. If you cache a translated
+  string in a plain field it WON'T update on switch; call `t()` in the template
+  (or in a signal-reading getter) instead. Same for `i18n.date()/size()`.
+- Dictionary values are TS string literals: **French/other apostrophes break
+  single-quoted entries** (`'…c'est…'`). Use double quotes for any value
+  containing an apostrophe. Plurals are `{ one, other }` maps (not ICU strings);
+  zh/ja supply only `other`.
+- **Native menus can't use the Angular service** — `menu.ts`/`context-menu.ts`
+  go through backend `i18n.ts`, and the app menu must be rebuilt when the locale
+  changes (wired in index.ts `settingsChanged`), or it keeps the old language.
+- `eu`/`zh`/`ja` are AI drafts pending native review (see the i18n README).
 - **Silent auto-refresh must snapshot sessionId+cwd and discard a mismatched
   result.** A normal `refresh()` is masked by `busy` (input blocked), but the
   silent variant (poll/watch) runs while the user can navigate — without the