dashboard: serve the dashboard at /, fold kept state in as a tab, add a pages menu

Drop the H0M3 tile hub and the core page. The dashboard is now the index;
kept state is its fifth tab. A pages menu in the chrome links flow, stats,
API docs and the swarm UI. The swarm apex URL comes from
services.hyperhive.swarm.domain via HIVE_SWARM_PUBLIC_URL and the new
swarm_public_url field of the state snapshot.

nix/host-modules/hive-forge/service.nix

@@ -116,13 +116,13 @@ in
       example = "https://forge.example.com";
       description = ''
         Browser-facing forge URL the dashboard uses to build clickable
-        forge links (the H0M3 Forge tile, per-agent-row forge links) —
+        forge links (the per-agent-row forge links) —
         sourced into every agent container + hive-c0re as
         `HIVE_FORGE_PUBLIC_URL`.
 
         Defaults to `https://''${cfg.domain}`, the gateway vhost. When
         `null`, the dashboard **hides** forge links rather than
-        guessing one — see `docs/web-ui/dashboard.md::H0M3 page` for
+        guessing one — see `docs/web-ui/dashboard.md::Container row` for
         the rationale (a link built from the operator's own browser
         hostname + a container port is only an accident away from
         wrong on any deployment that isn't plain localhost).