Closes #630. The wire-side half landed in 1.6.0 via PR #648
(documenting LocaleMiddleware so the API's `gettext_lazy`-wrapped
`verbose_name` / `help_text` resolve per the active locale). This
PR ships the SPA-side half: the chrome's own strings ("Add",
"Save", "Search", "Loading…") now flow through a message catalog
that hydrates from the request's active locale.

## How it works

1. **Server**: ``SpaIndexView`` resolves the active language via
   ``translation.get_language()`` — falls back to
   ``settings.LANGUAGE_CODE`` when ``LocaleMiddleware`` isn't in the
   stack. The shell template renders it as
   ``<meta name="dar-language">``.

2. **SPA boot**: ``main.tsx`` reads the meta tag and calls
   ``loadCatalog(language)`` BEFORE the React root mounts — so the
   first paint already carries translated strings (no FOUC, no
   fetch / loading state).

3. **Component use**: any package imports ``t`` from ``@dar/ui`` and
   wraps user-visible strings:

   ```tsx
   import { t } from '@dar/ui';
   <Button>{t('Save')}</Button>
   ```

   ``t(en)`` looks the English source up in the active catalog;
   missing key → returns ``en`` itself. Source-as-key (gettext
   convention) so refactoring a JSX string doesn't require a
   parallel catalog-key change, and gradual coverage degrades
   gracefully.

## Catalogs shipped

JSON catalogs bundled in the SPA build under
``packages/ui/src/i18n/``:

- ``en.json`` — placeholder; English round-trips via source-as-key.
- ``es.json`` — Spanish (40 keys).
- ``pt.json`` — Portuguese / pt-br (40 keys; ``pt-br`` aliased).
- ``fr.json`` — French (40 keys).

The set covers the high-visibility chrome strings — Add, Save,
Edit, Delete, Cancel, Refresh, Loading…, Clear all, Customize
columns, Layout, Sign in / out, History, the shuttle widget's
Available / Chosen / Choose all / Remove all / Filter, the
list-action redirect banner, the raw_id lookup link, etc.
Adding a new language: drop a JSON file, import it in
``i18n.ts``, ship.

## Tests

- ``frontend/packages/ui/src/i18n.test.ts`` — 14 cases pinning
  source-as-key fallback, exact + stem language matching
  (``es-AR`` → ``es``), case-insensitive code, unknown-code
  no-op, sanity-check that ES / PT / FR actually translate the
  expected keys, English source unchanged.
- ``tests/test_active_language.py`` — 3 cases pinning that the
  ``<meta name="dar-language">`` tag is emitted, that it follows
  ``translation.activate()``, and that it falls back to
  ``LANGUAGE_CODE`` when no locale is active.

## Coverage strategy

Wired ``t(…)`` into ``ShuttleSelect`` (the high-traffic new widget
from #627) as the proof-of-life. Remaining JSX strings stay English
until reached — each ``t(en)`` migration is independent and ships
incrementally. The README's i18n section (1.6.0) already explains
the pattern for consumers.

## Verification

- ``poetry run pytest -q`` — **64 / 64 ✓** (up from 61; +3 new in
  ``test_active_language.py``)
- ``pnpm test`` — **216 / 216 ✓** (up from 202; +14 new in
  ``i18n.test.ts``)
- ``pnpm -r typecheck`` ✓
- ``pnpm lint`` ✓
- ``pnpm -w build`` ✓

## Minor bump rationale

``1.6.0`` → ``1.7.0``. New user-visible capability (chrome
translation) per SemVer's "additive features" guideline. Pure
additive — English shops see no behaviour change.

Closes #630.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>