Repair external node links and deeplink the live coverage overlay

[SimmerV]

fix(ui): repair dead and stale external node links

All four default "Elsewhere" links on the node panel were broken in some way:

• Armooo's MeshView removed everywhere (sidebar tools, elsewhere defaults,
  config.toml.sample) — the instance is offline.
• Bay Mesh Explorer now points at the new instance at meshview.bayme.sh,
  which deeplinks by decimal node id: https://meshview.bayme.sh/node/{node_id_int}.
• Liam's Map / MeshMap templates were already correct but rendered as
  ?node_id=NaN on the map panel: node ids arriving with a ! prefix broke
  parseInt(id, 16). The panel now uses convertNodeIdFromHexToInt, and
  resolveElsewhereUrl also strips ! from {node_id_hex} substitutions for
  custom-configured templates.

Note for operators: instances that set mesh.tools / mesh.elsewhere_links in
their own config.toml override these defaults and need the same URL updates
(the Discord bot's "View Elsewhere" field reads the same config section).

feat(map): deeplink the live coverage overlay via ?cov=

Toggling the coverage pill now mirrors into the map URL (cov=1|0) so a shared
link reproduces the sharer's overlay state alongside the existing lat/lng/z:

• The param overrides the viewer's saved preference for that session without
  overwriting it (same principle as the LOS share links).
• Pristine URLs are never stamped with cov=0 — the param only appears once
  the overlay has been on; an explicit cov=0 is kept so links shared by
  someone with the overlay off still override a viewer whose default is on.
• cov=true/false accepted on read, normalized to 1/0 on write.
• Written via history.replaceState like the pan/zoom sync — toggling never
  re-renders the map route. Composes with ?node= and ?tool= links.

Parse/write rules are pure helpers in coverageUrlParam.ts with unit tests.

Testing

• 211/211 frontend tests pass (8 new for the ?cov= param semantics)
• tsc and eslint clean
• Deployed on a live instance: verified the corrected links resolve, the ?cov=
  param round-trips through share links, and it survives pan/zoom URL rewrites