Onboarding at events: translate /onboard, use "community" wording, and handle failed invite lookups #653

Ported from NEARBuilders/citynode.app#386 (open), where this was fully implemented in PR NEARBuilders/citynode.app#389 before the local auth-ui plugin was removed in favor of this repo's published auth plugin.

The reference implementation (diffs, tests, i18n keys) is preserved on that PR branch: fix/386-onboard-i18n-lookup-errors — files originally lived under plugins/auth-ui/src/ here (now plugins/auth/ui/src/).

Problem

plugins/auth/ui/src/routes/_public/onboard.tsx — the /onboard?code= page used at events — has gaps for new community members:

1. English only. All strings are hardcoded. The auth.onboard.* translation keys do not exist in this repo yet (citynode's old plugins/auth-ui/src/i18n/messages/auth.ts had full en/es/fr/zh coverage; this repo's plugins/auth/ui/src/i18n/catalogs.ts has no onboard keys).
2. Wrong wording. The page says "organization" instead of "community" ("Join the organization", "Could not join this organization").
3. Stuck loading. If the invite-code lookup fails (network error, 429, 5xx), the page shows "Loading invitation…" forever — no error state, no retry. A failed join ends on a panel whose only action is "Explore communities".

Proposed fix (mirroring the reference implementation)

• Add auth.onboard.* keys to the i18n catalogs (en/es/fr/zh) and render the onboard page (and its sign-up / display-name steps) through the login i18n runtime (LoginI18nProvider + useLoginTranslation in plugins/auth/ui/src/i18n/runtime.tsx).
• Replace "organization" with "community" everywhere in the flow.
• Map lookup/join failures to distinct status panels with reasons and next steps: lookup error → Retry (not an endless spinner); join failures → communityNotOpen, communityFull, codeRevoked, codeExpired, codeUsedUp, codeInvalid copy with "contact the organizer" / "explore" actions.
• Port the vitest coverage from plugins/auth-ui/src/routes/_public/-onboard.test.tsx on the reference branch (568 lines: language rendering, retry paths, join-failure panels).

Acceptance criteria

• /onboard?code= renders in es/fr/zh per the selected language.
• No "organization" wording remains in the onboarding flow.
• A failed invite lookup shows an error with a Retry button instead of an endless spinner.
• Failed joins show a specific reason and next step.