feat(i18n): multi-language support (en/ja/de/es) foundation

[JamieRuderman]

Summary

Adds a working multi-language (i18n) foundation across all three platforms (web portal, Electron desktop, Capacitor mobile) plus the Electron main process. Supports English (source/fallback) + Japanese, German, Spanish, with a language picker and OS-based auto-detection.

Because untranslated strings fall back to English, the app is shippable at every step — this PR lands the full infrastructure and a proven extraction template; the remaining bulk string extraction can follow incrementally.

What's included

• Core infra — app-wide i18next instance (frontend/src/i18n/) with app/notices/cognito namespaces. English bundled eagerly; ja/de/es lazy-loaded and code-split per locale. The existing Cognito auth module is folded into the shared instance.
• Language preference — language state in models/ui.ts mirroring the existing themeMode pattern (localStorage, persists across logout). Default system follows the OS/browser/device language via navigator.language, re-resolved each launch.
• Auto-detection — system (the default) matches the OS language on all three platforms; region tags degrade gracefully (ja-JP→ja, unknown→en). Standard "follow OS + in-app override" convention.
• Picker UI — selector in OptionsPage.tsx with native endonyms (日本語, Deutsch, Español).
• Electron main — tray, menus, and native dialogs localized (electron/src/i18n.ts + electron/src/locales/*.json); language synced from the renderer through the existing preferences channel (no new IPC).
• Dates/durations — humanizeDuration() + getLocale() in dateHelper.ts; luxon locale follows the app language.
• Tooling — npm run i18n:extract (i18next-parser) and npm run i18n:check (CI key-parity / empty-English gate).

Cognito auth and OptionsPage are fully translated in all four languages (machine-seeded, pending professional review before production promotion).

Verified

• Type-check clean (frontend + electron), production build passes with per-locale code-splitting.
• Runtime: live language switching confirmed in a browser harness — interpolation ({{device}}, {{version}}) renders correctly in Japanese with adapted word order; unseeded keys fall back to English (returnEmptyString: false).
• i18n:check passes across all 4 locales / 3 namespaces.

Remaining (follow-up work)

• Bulk extraction of ~570 more .tsx files (order: shell/nav → core surfaces → ~41 redux-model notice messages with plural fixes → long tail). Each batch: extract → parser → Sonnet-seed → parity gate.
• Pseudo-locale + key-parity CI job; full manual matrix (Electron tray, iOS simulator).

Decisions worth noting

• Strict TS key-typing rejected — it forces t() return types into a union that breaks existing string usages and dynamic keys; the CI parity check covers correctness instead.
• Parser set to never auto-prune — some cognito keys are built dynamically and would otherwise be silently deleted.

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