Render breadcrumbs from the shell via route handles (useMatches) so skip-to-main clears them

## Context

PR #157 wired `Breadcrumbs.tsx` into the six screens that `specs/behaviors/app-shell.md` names. Each screen renders `` itself as a fragment sibling above its content container, because that was the minimal change that made the spec's trail table true.

## Problem

`app-shell.md` describes the breadcrumb row as part of the *shell* ("an optional row below the header ... a screen opts in by declaring a `breadcrumbs` trail"), and the shell's skip link targets ``. With the crumbs rendered inside each screen, "skip to main content" lands on the breadcrumb nav rather than past it, and every screen repeats the same import + placement.

## Proposal

Render breadcrumbs once from `AppShell`, sourced from route `handle`s via `useMatches()` (React Router v7). Each opted-in route declares its trail on its `handle` (with loader/query data supplying the dynamic segment), and the six per-screen `` calls go away. The row then sits between the header and ``, so the skip link clears it.

Scope: `apps/web/src/components/AppShell.tsx`, the router config, and the six screens (`ProjectDetail`, `ProjectEdit`, `PersonDetail`, `TagDetail`, `Account`, plus the create variant). Tests in `ProjectDetail.test.tsx` / `PersonDetail.test.tsx` assert the trails and should keep passing.

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