feat(meta): gate deferred Fleet tabs behind SENCHO_EXPERIMENTAL flag (#886)

Hide the Traffic / Routing, Deployments, Federation and Secrets Fleet
tabs by default. They re-appear when the operator opts in by setting
SENCHO_EXPERIMENTAL=true. Backend routes and database tables are
unchanged; this is a UI discovery gate only.

The /api/meta endpoint now returns experimental as a boolean. A new
useExperimental hook reads it once per page load and feeds the four
tab triggers and tab content panels in FleetView.

frontend/src/hooks/useExperimental.ts

@@ -0,0 +1,52 @@
+import { useEffect, useState } from 'react';
+import { apiFetch } from '@/lib/api';
+
+// Module-scope cache: read once at boot, do not invalidate. The
+// SENCHO_EXPERIMENTAL flag is read from the gateway node's process
+// env at request time, so it cannot flip mid-session without a
+// restart. If the initial fetch fails the value sticks at false until
+// a full reload; that is acceptable for a dev-only flag.
+let cached: boolean | null = null;
+let inflight: Promise<boolean> | null = null;
+
+async function fetchExperimental(): Promise<boolean> {
+  if (cached !== null) return cached;
+  if (inflight) return inflight;
+  inflight = (async () => {
+    try {
+      // localOnly: the flag is a property of the gateway running the
+      // browser session, not of whichever remote node is currently
+      // selected. Without this, switching nodes would re-evaluate the
+      // flag against the wrong process env.
+      const res = await apiFetch('/meta', { localOnly: true });
+      if (!res.ok) {
+        cached = false;
+        return false;
+      }
+      const body = (await res.json()) as { experimental?: boolean };
+      const next = body.experimental === true;
+      cached = next;
+      return next;
+    } catch {
+      cached = false;
+      return false;
+    } finally {
+      inflight = null;
+    }
+  })();
+  return inflight;
+}
+
+export function useExperimental(): boolean {
+  const [value, setValue] = useState<boolean>(cached ?? false);
+  useEffect(() => {
+    let active = true;
+    fetchExperimental().then((next) => {
+      if (active) setValue(next);
+    });
+    return () => {
+      active = false;
+    };
+  }, []);
+  return value;
+}