RelativeTime component for the 13 <time dateTime title> sites

## Context

PR #157 converted every relative or `title`-only date in `apps/web` to `{formatRelativeTime(iso)}` — 13 sites after the sweep, across `ActivityCard`, `HelpWantedCard`, `BlogIndex`, `BlogDetail`, `Account`, `AccountClaim`, `StaffAccountClaimQueue`, `PersonDetail`, `ProjectDetail`.

## Problem

Three attributes and two formatter calls per site, all identical. The next date rendered in the app will likely drop one of them (that is exactly how three sites were missed on the first pass of #157).

## Proposal

A `RelativeTime` component (`apps/web/src/components/RelativeTime.tsx`) taking `iso` and an optional `className`, rendering the `` with `dateTime`, the absolute `title`, and the relative text; optionally a `prefix` prop for "posted", "joined", etc. Migrate the 13 sites.

Deferred from `plans/a11y-mechanical.md` (PR #157).

Start with apps/web/src/components/RelativeTime.tsx and review PR #157 plus plans/a11y-mechanical.md for the existing date patterns. Check the 13 named sites across ActivityCard, HelpWantedCard, BlogIndex, BlogDetail, Account, AccountClaim, StaffAccountClaimQueue, PersonDetail, and ProjectDetail. Done means those sites use the shared component with their needed props and retain the dateTime, absolute title, relative text, and any prefixes.

索引モデルが issue の本文から書いたものです。