feat(i18n): mount the provider and surface the language switcher (#159)

[devrim-1283]

Closes #159.

The i18n work was all present and none of it reachable. I18nContext, the en/es locale tables and LanguageSwitcher existed — LanguageSwitcher even carries a closes #159 comment — but:

$ grep -rn "I18nProvider" src/ --include=*.tsx | grep -v context/I18nContext
(no matches)
$ grep -rn "LanguageSwitcher" src/ --include=*.tsx | grep -v components/LanguageSwitcher
(no matches)

Nothing mounted the provider and nothing rendered the switcher. No user could change language, and useI18n() would have thrown anywhere it was tried.

Wiring

• I18nProvider wraps the tree in context/provider.tsx, inside QueryClientProvider and around StellarNetworkProvider.
• LanguageSwitcher renders in TopHeader, next to the role badge.

Two defects found while wiring it

1. Prototype-chain lookup. readPersistedLocale used stored in locales, and in walks the prototype chain:

localStorage.setItem("audioblocks_locale", "constructor");
// "constructor" in locales  ->  true
// locales["constructor"]    ->  Object.prototype.constructor, a function

So t became a function instead of a translation table and every label in the UI rendered undefined. Now Object.hasOwn. setLocale gained the same validation, so an unsupported value cannot be written in the first place.

2. Unguarded storage access. Safari private mode and blocked site data both throw on localStorage, which would have taken the whole provider down on mount. Both calls are now wrapped.

Keeping existing tests working

23 existing tests render TopHeader without a provider. Rather than rewrite them, useOptionalI18n() was added for components that are i18n-aware rather than i18n-dependent. LanguageSwitcher uses it and renders null when there is no provider — it has nothing to switch. useI18n() still throws, so a component that genuinely needs translations fails loudly.

Tests

15 new cases: default locale, persistence and restore, an unsupported locale ignored, the four inherited-property values rejected, a localStorage that throws on read and on write, useI18n outside a provider, the switcher listing and changing locales, and a parity check that en and es define exactly the same keys with no empty strings.

Verified the tests catch the bug: restoring in turns 4 of them red.

No regressions

	Tests
before	26 failed, 381 passed
after	26 failed, 396 passed

Same 26 failures, and the delta is the 15 new tests. tsc --noEmit reports the same 181 pre-existing errors. The one eslint error in I18nContext.tsx (react-hooks/set-state-in-effect) is pre-existing too — it is the SSR-safe mount effect, unchanged here.

Reproducing locally needs npm install --legacy-peer-deps.

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