ExternalLink component for the 16 hand-rolled "(opens in new tab)" links

## Context

PR #157 added a new-tab cue to every `target="_blank"` link in `apps/web`: ` (opens in new tab)` where the link has visible text, or appended to the `aria-label` where it does not. That is 16 copies across 9 files (`AppFooter`, `AppHeader`, `ProjectCard`, `ActivityCard`, `LoginPlaceholder`, `Account`, `PersonDetail`, `ProjectDetail`, `Volunteer`).

## Problem

The cue text, the `rel="noopener noreferrer"`, and the `target` are re-typed at every site. A future link will forget one of them, and changing the wording (or adding a visual icon) means touching 16 places.

## Proposal

An `ExternalLink` component in `apps/web/src/components/` that takes `href` + children, sets `target`/`rel`, and appends the sr-only cue (or merges it into a supplied `aria-label`). Migrate the 16 sites; the existing accessible-name assertions in the test suite (regex matchers on `/\(opens in new tab\)$/`) cover the migration.

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

从 apps/web/src/components/ 开始，检查 AppFooter、AppHeader、ProjectCard、ActivityCard、LoginPlaceholder、Account、PersonDetail、ProjectDetail 和 Volunteer 中的 16 个 target links。运行现有的 accessible-name 测试，然后确认每个 site 都使用共享组件，并且 assertions 仍与 new-tab cue 以及 target 和 rel 的行为相匹配。

由索引模型根据 Issue 内容生成。