fix(person-mention): name the hover-card region and announce its busy state

Leonie FINDING-02/03 + Elicit NFR concern + Sara #4: role="region" with no
aria-label is an axe-core warning, and the pulsing-bars skeleton carries no
semantics for SR clients.

- Add aria-label to the region root: person displayName when loaded,
  localised "Lade Person…" while loading. Region always has a name.
- Add aria-busy="true" while loading; cleared on loaded/error so the
  state change is announced via aria-live="polite".
- Add role="status" + aria-label on the skeleton so SR clients hear
  "Lade Person" rather than three silent <div>s.
- New Paraglide key person_mention_loading in de/en/es.

Five new tests pin: aria-busy true while loading, aria-busy unset/false
when loaded, aria-label is displayName when loaded, aria-label is the
loading label while loading.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -423,6 +423,7 @@
 	"person_mention_open_link": "Zur Person",
 	"person_mention_hover_hint": "Klick öffnet Seite",
 	"person_mention_load_error": "Person konnte nicht geladen werden.",
+	"person_mention_loading": "Lade Person…",
 	"person_mention_popup_empty": "Keine Personen gefunden",
 	"person_mention_btn_label": "Person verlinken",
 	"person_mention_create_new": "Neue Person anlegen",