fix(frontend): create the map once instead of re-initializing on every node update

[SimmerV]

Problem

The map randomly flickers, reloads its basemap tiles, and sometimes snaps the
camera back while you're using it. Trigger: any node-list refresh (initial load,
refetch on tab focus/reconnect) tore down and rebuilt the entire MapLibre
instance — reloading tiles and resetting unsaved zoom/pan/pitch.

Root cause

The map-init effect was keyed on serverNode, whose identity changes on every
getNodes cache update, and its cleanup calls map.remove() — so the whole map
was recreated each time.

Fix

Create the map once. Read the server node via a ref for the initial center, and
recenter once on load via a one-time jumpTo instead of a full re-init.

• closes Bug(map): flickers on initial load #485