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-c0re/src/dashboard.rs

@@ -250,6 +250,12 @@ struct StateSnapshot {
     /// local-dev deploys keep working. See `docs/gateway.md::Vhost
     /// map`.
     gateway_enabled: bool,
+    /// Public URL of the forge vhost served by hive-gateway (e.g.
+    /// `"http://forge.pr1ma.darkest.space"`). Sourced from the
+    /// `HIVE_FORGE_PUBLIC_URL` env var, which the c0re NixOS module
+    /// sets when `forge.behindGateway = true`. `None` when absent —
+    /// the frontend falls back to `http://<hostname>:3000`.
+    forge_public_url: Option<String>,
     /// Human name of this single-host hive instance (e.g. `"pr1ma"`).
     /// Sourced from `HYPERHIVE_HIVE_NAME` env var, set by the c0re
     /// NixOS module from `services.hyperhive.hiveName`. `None` when
@@ -466,6 +472,7 @@ async fn api_state(headers: HeaderMap, State(state): State<AppState>) -> axum::J
             let s = v.to_string_lossy().to_ascii_lowercase();
             matches!(s.as_str(), "1" | "true" | "yes")
         }),
+        forge_public_url: std::env::var("HIVE_FORGE_PUBLIC_URL").ok().filter(|s| !s.is_empty()),
         hive_name: std::env::var("HYPERHIVE_HIVE_NAME").ok().filter(|s| !s.is_empty()),
         swarm_name: std::env::var("HYPERHIVE_SWARM_NAME").ok().filter(|s| !s.is_empty()),
     })