Fix ADMIN_PANEL_ENABLED never being read from frontend/.env

process.env.ADMIN_PANEL_ENABLED was always undefined in the running
SvelteKit server process — Vite only injects VITE_-prefixed vars into
process.env for server-side code; plain vars in frontend/.env never reached
it, so the admin panel stayed disabled (cog hidden, /admin/* 404s) no matter
what the .env file said. Switched to SvelteKit's own $env/dynamic/private,
which reads it correctly in dev, preview, and adapter-based deployments.

Reproduced and verified the fix against a real frontend/.env file (not an
inline shell var, which is what masked this the first time).

frontend/src/routes/+layout.server.ts

@@ -1,9 +1,16 @@
+import { env } from '$env/dynamic/private';
 import type { LayoutServerLoad } from './$types';
 
 // The admin panel is off by default on every deployment — it only appears (cog icon
 // and the /admin/* pages themselves, see admin/+layout.svelte) once this is
 // explicitly turned on. This is a UI-visibility gate only; the backend's API key
 // check on every /api/admin/* request is what actually protects it either way.
+//
+// Uses $env/dynamic/private rather than raw process.env — Vite does not inject
+// arbitrary (non-VITE_-prefixed) .env vars into process.env for server code, so
+// process.env.ADMIN_PANEL_ENABLED is always undefined here even with it set in
+// frontend/.env. $env/dynamic/private is SvelteKit's own env accessor and reads it
+// correctly in dev, preview, and any adapter-based deployment.
 export const load: LayoutServerLoad = async () => {
-	return { adminPanelEnabled: process.env.ADMIN_PANEL_ENABLED === 'true' };
+	return { adminPanelEnabled: env.ADMIN_PANEL_ENABLED === 'true' };
 };