feat(mesh): UI for the gateway-to-gateway mesh

The mesh API (routes/mesh.js) had zero UI -- minting a join token,
joining a remote gateway, or seeing what's meshed all required calling
the API directly. New Mesh page (nav: Dashboard/Sessions/WireGuard/
Mesh/Audit):

- This Gateway card: interface name, kernel-vs-userspace WireGuard mode
  (wireguard-go fallback), meshed-gateway count.
- Mint a Join Token: calls POST /api/mesh/join-tokens, shows the
  single-use token once.
- Join a Remote Gateway's Mesh: calls POST /api/mesh/join with a remote
  endpoint + token.
- Meshed Gateways table: site, mesh index, mesh subnet, endpoint, public
  key, last seen -- including this gateway's own self-entry.

EJS compile verified; jump-host's existing test suite (34 tests) still
passes. Not yet visually driven in a browser the way sso-manager-node's
modal was (jump-host's OIDC-based admin auth is a heavier lift to stand
up for a one-off check) -- route registration, EJS compilation, and the
API layer underneath are verified; the actual click-through is not.

nodejs/routes/render.js

@@ -48,5 +48,6 @@ router.get('/dashboard', (req, res) => res.render('dashboard', {...values}));
 router.get('/sessions', (req, res) => res.render('sessions', {...values}));
 router.get('/audit', (req, res) => res.render('audit', {...values}));
 router.get('/wireguard', (req, res) => res.render('wireguard', {...values}));
+router.get('/mesh', (req, res) => res.render('mesh', {...values}));
 
 module.exports = router;