feature: i18n/multi-language support for the frontend

[tech-adrian]

Problem

frontend/src/app/** and frontend/src/components/ui/** have all user-facing copy (labels, error text from friendlyError(), ProgressSteps step names, etc.) written as inline English strings — there's no extraction layer or locale switching, so the app is only usable in English.

Why it matters

A shielded wallet aimed at consumer-grade UX and broad adoption (per README.md) is unnecessarily limited to English-speaking users; this is a straightforward, well-understood gap with no cryptographic or protocol risk, making it a good substantial contribution for someone building frontend familiarity with the codebase.

Scope

• Add an i18n library (e.g. next-intl or react-i18next) as a frontend/package.json dependency.
• New frontend/src/messages/en.json plus at least two additional locales (e.g. es.json, fr.json) covering every user-facing string.
• Extract inline strings from frontend/src/app/deposit/page.tsx, withdraw/page.tsx, compliance/page.tsx, history/page.tsx, and frontend/src/components/ui/*.tsx into the message catalog.
• frontend/src/lib/errors.ts: route friendlyError() messages through the same catalog so translated errors stay in sync with translated UI.
• New locale-switcher component in the UI (e.g. added near existing header/nav elements).
• frontend/src/middleware.ts (new, if using a routing-based i18n library) or equivalent config for locale detection/routing.
• Tests: a snapshot/lint check that no new hardcoded English string lands outside the message catalog (e.g. an ESLint rule or a script check wired into CI).

Acceptance criteria

• The full deposit → withdraw → compliance flow is usable in at least two non-English locales with no untranslated strings visible.
• A CI check catches new hardcoded strings added outside the message catalog going forward.

[Fabluchy]

Hi maintainer please assign it to me

[dimin4241-svg]

Hi, I’d like to work on this. I have shipped Next.js/TypeScript UI and regression-test changes, and I understand this needs one consistent catalog across pages, shared UI, and friendlyError(), plus locale switching and a CI guard against new inline copy. I can deliver en/es/fr coverage and focused tests in 1–2 days. Please assign me.