dashboard: hide forge links instead of guessing <hostname>:3000

Adds services.hyperhive.forge.publicUrl (defaults to the gateway vhost
URL when behindGateway=true, null otherwise). HIVE_FORGE_PUBLIC_URL is
now sourced from it instead of hardcoding https://${forge.domain}
whenever behindGateway is on.

The 4 frontend call sites that built a forge link from
state.forge_public_url now hide the link when that's absent, rather
than guessing http://<browser-hostname>:3000 — a guess that's only
correct by accident once the operator isn't on plain localhost. Fixes
the dashboard H0M3 tile, per-agent-row forge links + agent menu, the
approval-queue PR link, and the per-agent page's own meta-nav forge
link (found during this pass, same defect, not in the original
3-site inventory).

Docs + doc-comments updated to match.

hive-c0re/src/dashboard/state_snapshot.rs

@@ -107,11 +107,15 @@ pub(super) struct StateSnapshot {
     /// `http://<hostname>:<port>/` TCP links) is retained as a defensive
     /// fallback for the env being unset. See `docs/gateway.md::Vhost map`.
     gateway_enabled: bool,
-    /// Public URL of the forge vhost served by hive-gateway (e.g.
+    /// Public URL of the forge (e.g.
     /// `"https://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`.
+    /// sets from `services.hyperhive.forge.publicUrl` (defaults to
+    /// the gateway vhost URL when `forge.behindGateway = true`,
+    /// `null` otherwise). `None` when absent — the frontend **hides**
+    /// forge links rather than guessing `http://<hostname>:3000`,
+    /// which is only right by accident on deployments that aren't
+    /// plain localhost.
     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