fix(settings): translate join-request approve/deny toasts

[pedrofrxncx]

The admin billing page (#5863) is still an open PR — nothing on main under apps/web/src/routes/admin lands billing yet, so this focuses on the adjacent components/settings surface (join-requests-section.tsx, used on the org members admin page for domain-join approvals).

Bug: useJoinRequestActions in use-join-requests.ts hardcoded the approve/deny success and fallback-error toast strings in English ("Request approved", "Request denied", "Failed to approve", "Failed to deny"), bypassing the repo's i18n system even though the surrounding join-requests-section.tsx component and its dictionary keys (settings.joinRequestsSection.*) already exist and are threaded through t(). A pt-br user approving/denying a join request saw English toasts.

Fix: added approved/denied/approveFailed/denyFailed keys to en/settings.ts and pt-br/settings.ts, and routed the four toast calls through useT().

How to confirm: switch language to pt-br in preferences, approve or deny a pending join request on the org members page, see the toast in Portuguese.

Checks run locally: bun run fmt, cd apps/web && bunx tsc --noEmit (clean), bunx oxlint on the three touched files (0 warnings/errors). No test needed — this is a pure string/translation-plumbing change, not new logic. Full CI validates the rest.

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Summary by cubic

Localizes join-request approval/denial toasts. Previously success and fallback error toasts were hardcoded in English; now they use existing i18n keys so users see messages in their selected language (including pt-BR).

• Route toast messages in useJoinRequestActions through useT(). Use the server error message when present; otherwise use translated fallbacks.
• Add approved, denied, approveFailed, and denyFailed keys to en/settings.ts and pt-br/settings.ts.
• No logic changes to mutations or cache invalidation; no migration required. QA: set language to pt-BR, approve/deny a join request, verify localized toasts.

^{Written for commit 3748d7c. Summary will update on new commits.}