feat: add Estonian (et) locale

[raudraido]

Summary

Adds Estonian as a fully supported locale, following the same pattern used when Polish was added:

• Full translation of locales/et/common.json (same key set as every other locale, verified by the parity test).
• Registered in i18n/routing.ts, components/ui/language-switcher.tsx (native label Eesti), and components/providers/intl-provider.tsx (pre-loaded message bundle).
• Every existing locale gets language.estonian / language.switch_to_estonian entries, mirroring the existing convention where a language's own native name (e.g. "polish": "Polski") is identical across every bundle, while switch_to_X is translated into the viewing locale's own language.
• lib/__tests__/locale-parity.test.ts updated: imports the new bundle, bumps the expected locale count to 12, and adds an Estonian-labels check mirroring the existing Polish one.

Also included, found while preparing/testing this change:

• A one-line fix: for npm run lint, which is currently broken on main — Next.js 16 removed the next lint command entirely, but package.json still calls it (CONTRIBUTING.md asks every contributor to run this before opening a PR).
• A fix: for several strings that bypass useTranslations() entirely and always render in English regardless of locale, found by testing the new Estonian translation end-to-end:
  • Reply / Reply all / Forward labels in the email viewer were hardcoded JSX text.
  • System mailbox names (Inbox, Drafts, Sent, Trash, Archive, Junk, ...) are rendered from the JMAP mailbox's own name everywhere (sidebar, mobile header, tab title, "Move to..." popover, context-menu submenu) instead of the already-translated sidebar.mailboxes.* labels that existed in every locale but were never used. Added a shared getMailboxDisplayName(role, name, t) helper and applied it everywhere a mailbox name is shown, including the move-to-popover's search filter.
  • The open-email header timestamp hardcoded toLocaleString('en-US', ...); the reply-quote header used the browser/OS default locale instead of the app's selected one.
  • formatDate() (the relative "2h ago" timestamps in every email list row) hardcoded English strings and en-US, even though translated date.just_now / date.minutes_ago / date.hours_ago / date.days_ago keys already existed in every locale and were simply never called.
  • A few list-item components hardcoded "(no subject)" / "No preview available" / "Unknown" instead of using the equivalent keys that already existed elsewhere (added email_list.no_preview to every locale since no existing key matched that exact string).
• A fix: for a real race condition in IntlProvider that made the selected locale silently revert to the server default on every reload. zustand's persist middleware rehydrates from localStorage asynchronously (even for a synchronous backend like localStorage), so currentLocale can still read as its pre-hydration null for the first tick after mount. The effect that seeds the store with the server-rendered default locale ran unconditionally whenever currentLocale was falsy — including that pre-hydration tick — so it would frequently win the race and overwrite a real saved locale with en before the persisted value ever loaded. Fixed by tracking hydration explicitly via persist.hasHydrated()/onFinishHydration() and only seeding the default once hydration is confirmed complete.
  • Follow-up fix in the same area: persist.hasHydrated/onFinishHydration are only present in a browser context — during Next.js SSR there's no localStorage and .persist itself is undefined, so calling it unconditionally 500'd on every server render. The vitest suite didn't catch this since it runs in jsdom, where .persist always exists. Guarded with optional chaining; a missing .persist is treated as "already hydrated" (correct for SSR — nothing to wait for).

Test plan

• npx vitest run — 877/877 passing.
• npx eslint . — clean, no warnings.
• tsc --noEmit — clean.
• npx next build --webpack — builds clean.
• Manually verified in a real deployment: language switcher shows "Eesti", selecting it translates the full UI including mailbox names/dates/reply buttons, reloading persists the choice, and the app actually renders (caught the SSR crash live before this fix landed).