fix: redirect the pre-2.0.0 singular route paths (#85491)

[volarname]

2.0.0 pluralized every route path and added no redirect layer, so links to admin-dam held by other
apps 404. A search through common-admin and admin-cms turned up three paths that are still linked
from outside:

• /asset/:id — built by DamAdminAssetLink.vue in common-admin, rendered by admin-cms (audio
  widget, audio/video embed dialogs), by common-admin itself (image detail dialog) and inherited by
  admin-ugc.
• /asset/file/:id — asset-file detail, linked from an external library.
• /user/:id/edit — admin-cms DAM_USER_EDIT_URL_TEMPLATE, opened by SystemUserEditButton.vue.
  Its value lives in .env, every developer's .env.local, the generated public/config.json and
  the deployment pipeline variables, so it outlives a single code change.

They are added with router.addRoute() and re-added inside the handleHotUpdate callback, because a
hot update replaces the generated routes and drops anything added at runtime. Redirect targets are
route names rather than paths, so the anzu-local/valid-route-name lint rule fails loudly if a
page file is renamed instead of the shim silently redirecting to a 404.

The shim is temporary — the TODO in src/router/legacyRedirects.ts lists what has to change before
it can be removed.

Verification

• yarn lint clean (vue-tsc, oxlint, eslint, stylelint).
• Navigated a memory-history router: all three legacy paths redirect to the plural path with params,
  query and hash preserved (/asset/abc?tab=meta#roi → /assets/abc?tab=meta#roi), and the plural
  paths are unaffected.
• nginx only special-cases ^/assets/?$, so the legacy paths fall through the default try_files to
  index.html and reach the SPA.

Released as 2.0.1 — see doc/changelog/2.0.1.md.