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

hive-ag3nt/src/web_ui.rs

@@ -436,6 +436,12 @@ struct StateSnapshot {
     /// this directly. See [`docs/web-ui/dashboard.md::Container row`] for the
     /// frontend resolver + which links appear in which conditions.
     links: Vec<AgentLink>,
+    /// Public URL of the forge served by hive-gateway (e.g.
+    /// `"https://forge.pr1ma.darkest.space"`). Sourced from
+    /// `HIVE_FORGE_PUBLIC_URL`; `None` when `forge.behindGateway=false`
+    /// or the env var is absent. The frontend uses this to build forge
+    /// nav-strip links instead of hardcoding `<hostname>:3000`.
+    forge_public_url: Option<String>,
     /// Human name of this hive instance (e.g. `"pr1ma"`). Sourced
     /// from `HYPERHIVE_HIVE_NAME`; `None` when unset. The frontend
     /// uses this for the page `<title>` and header label so browser
@@ -585,6 +591,7 @@ async fn api_state(State(state): State<AppState>) -> axum::Json<StateSnapshot> {
         ctx_usage,
         cost_usage,
         links: agent_links(&state.label, state.gui_vnc_port.is_some()),
+        forge_public_url: std::env::var("HIVE_FORGE_PUBLIC_URL").ok().filter(|s| !s.is_empty()),
         hive_name: crate::identity::hive_name(),
         swarm_name: crate::identity::swarm_name(),
     })