feat: i18n, PWA offline mode, high-contrast theme & coverage CI gate (#1329, #1330, #1332, #1333)

[Hollujay]

Summary

Implements four frontend / CI issues in one PR.

Closes #1329
Closes #1330
Closes #1332
Closes #1333

#1329 – Multi-language i18n

• Lightweight dictionaries (no new dependency) in frontend/src/locales/ for en (default), es, fr, pt, zh, ja, covering Navbar, Dashboard, Stream Wizard, Settings, theme and offline strings.
• I18nProvider / useI18n() (context/i18n-provider.tsx): stored preference (flowfi-locale) → navigator.languages detection → English. It sets <html lang> and exposes t() with {var} interpolation plus locale-aware formatNumber, formatCurrency and formatDate built on Intl.
• LanguageSelector added to the Navbar (desktop and mobile menu) and Settings. The choice persists in localStorage.
• Navbar and Settings are wired to t(). The Dashboard and Stream Wizard keys exist in every dictionary, but those screens' own copy is not migrated in this PR.

#1330 – PWA & offline read-only mode

• public/manifest.webmanifest (standalone, theme colours, 192/512 and maskable icons), plus icons, and manifest, viewport and Apple metadata in layout.tsx.
• public/sw.js: stale-while-revalidate for static assets, fonts and API GETs; navigation falls back to cached pages when offline. SSE and auth requests are never cached. It is registered in production builds only.
• useNetworkStatus and OfflineBanner: a non-intrusive banner reading "Offline (Last synced X mins ago)". The sync time comes from successful service-worker API responses and reconnects.

#1332 – Theme architecture & high-contrast

• globals.css now has semantic tokens (--color-bg-canvas, --color-bg-surface, --color-border-subtle, --color-text-{primary,secondary,muted}, --color-brand-{primary,accent}). The legacy variables now read from them.
• New data-theme="high-contrast": black canvas, white text and borders, yellow (#ffff00) focus ring. Every text/background/border token pair is at least 15:1 on the black canvas (WCAG AAA needs 7:1). Hard-coded slate/zinc/white-alpha utility classes are remapped onto the tokens in this theme.
• No flash on reload: next-themes' pre-paint script now sets both class and data-theme. ModeToggle is a System / Light / Dark / High-contrast radio group, and Settings offers the same four options.

#1333 – Coverage enforcement & CI gate

• Frontend and backend Vitest configs emit lcov, json and json-summary. The frontend previously did not write lcov.info, which CI uploads.
• CI posts a per-file coverage table on PRs (vitest-coverage-report-action, changed files only). It is non-blocking so fork PRs with read-only tokens don't fail.
• codecov.yml: threshold: 1% on the backend, frontend and contracts project statuses, so a PR fails if coverage drops by more than 1%. It also adds flag carryforward and a per-file comment layout.
• Threshold note: the issue asks for 80/75/80/80. Measured frontend coverage today is 51% statements, 80% branches, 67% functions and 51% lines, so an 80% floor would fail every PR. The gate is set to a ratchet at the current baseline (50/75/65/50) with the 80/75/80/80 target documented in the config. Raise it as coverage improves. Backend thresholds are unchanged at 60%; I couldn't run the DB-backed backend suite locally to re-measure them.

Testing

• New tests: i18n.test.tsx (dictionary key and placeholder parity across locales, detection, persistence, formatting), theme.test.tsx, offline.test.tsx. All pass.
• tsc and ESLint report nothing in the files added or changed here.
• Full frontend suite: 27 failures, the same set as on unmodified main in my environment (stream-details-content, Navbar.test, TransactionTracker, and others). They are missing QueryClientProvider and jest globals, and were not caused by this change. next build also stops at type errors in files this PR doesn't touch (dashboard-view.tsx, StreamCreationWizard.tsx, soroban.ts).
• Not verified: Lighthouse PWA audit and installability on real devices, and the CI workflow changes. Both need a real deployment or a CI run.