fix(#888): use gateway forge URL in dashboard links

Dashboard links to the forge were hardcoded as http://<hostname>:3000,
which breaks when the operator accesses the dashboard through
hive-gateway (forge is served at forge.<domain> with no port).

- nix/modules/hive-c0re.nix: inject HIVE_FORGE_PUBLIC_URL when
  forge.behindGateway = true (e.g. http://forge.pr1ma.darkest.space)
- hive-c0re/src/dashboard.rs: expose forge_url: Option<String> in
  StateSnapshot, reading from HIVE_FORGE_PUBLIC_URL
- frontend/packages/dashboard/src/tabs.js: use state.forge_url when
  present; fall back to http://<hostname>:3000 for gateway-off /
  local-dev deploys

frontend/packages/agent/src/app.js

@@ -1063,7 +1063,9 @@ window.marked = marked;
       const metaLinks = $('meta-links');
       if (metaLinks && Array.isArray(s.links)) {
         metaLinks.replaceChildren();
-        const forgeBase = `http://${window.location.hostname}:3000`;
+        // Prefer forge_public_url from state (set when the gateway serves
+        // forge at its sub-domain); fall back to :3000 for local-dev.
+        const forgeBase = s.forge_public_url || `http://${window.location.hostname}:3000`;
         s.links.forEach((lnk, i) => {
           const href = lnk.kind === 'forge'    ? forgeBase + (lnk.url || '')
                      : lnk.kind === 'external' ? (lnk.url || '')