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.

backend/src/routes/meta.ts

@@ -22,6 +22,7 @@ metaRouter.get('/meta', (_req: Request, res: Response): void => {
     version: getSenchoVersion(),
     capabilities: getActiveCapabilities(),
     startedAt: processStartedAt,
+    experimental: process.env.SENCHO_EXPERIMENTAL === 'true',
     ...(updateError ? { updateError } : {}),
   });
 });