preact 04-login: xlogin + WebID fetch

  - useXlogin() hook: listens for 'xlogin'/'xlogout' events on document,
    late-load polls window.xlogin once every 500ms until the script
    attaches. Returns { type, id } or null.
  - fetchWebidProfile(): GETs the WebID URL with Accept: application/ld+json,
    uses xlogin.authFetch if present. Pulls the subject matching the
    fragment from a @graph if present. Normalizes common foaf/vcard
    predicates down to our bare keys (name, email, img, homepage, nick).
  - ProfileView overlays the fetched profile onto the sample island, with
    a small badge (live / loading / error / sample) above the hero.
  - Sidebar IdentityChip shows "Signed in · solid" + truncated id.
  - Settings surfaces the live identity alongside the localStorage fallback.

Author: melvincarvalho