feat(admin): deep-linkable console URLs, stop collapsing unknown paths to /admin

The root catch-all page ([...slug].vue) redirected every unknown URL to
/admin, and the worker fell back to the SPA for any path — so no matter
what page was opened, the URL ended up at baseurl/admin.

- Extract the console into ConsolePage.vue; the active tab now mirrors
  the URL path: /admin (groups), /admin/groups, /admin/logs, /admin/audit
  (tab switches use router.replace; unknown slugs render 404)
- admin.vue and admin/[slug].vue become thin wrappers
- Delete the root [...slug].vue catch-all redirect
- server.ts: notFound serves ASSETS (SPA + _nuxt chunks) only for
  /admin-prefixed paths; every other unknown URL returns a JSON 404

docs/guide/configuration.md

@@ -56,6 +56,8 @@ WebHooker ships with a built-in config console at `/admin` for managing routes i
 
 ### Endpoints
 
+The console is served as an SPA at `/admin`; its tabs are deep-linkable via the URL path (`/admin/groups`, `/admin/logs`, `/admin/audit`). URLs outside `/admin` that do not match an endpoint below return a plain `404` instead of the console.
+
 | Endpoint                                  | Description                                  |
 | ----------------------------------------- | -------------------------------------------- |
 | `GET /admin`                              | Config console UI                            |