Switch the Adverts and Messages observer filter from one badge per observer
node to one badge per unique area tag value (e.g. IP2, IP3, IP4). Toggling
an area badge enables/disables all observers in that area together.
Observers without an area tag are hidden from the filter row. Uses a new
localStorage key (meshcore-observer-areas-disabled) to avoid stale public
keys being misread as area codes. The old key is cleaned up on SPA boot.
Frontend-only: no backend, API, schema, or migration changes.