prices: fold groceries and CS skins into one price tracker (/trackers/prices) with tabs

- tab registry in #lib/price-trackers (add a tab = route + entry + icon)
- groceries -> /trackers/prices/groceries (owner-only), CS prices -> /trackers/prices/cs (public)
- old /trackers/groceries/* and /projects/cs-prices/* redirect (301 GET, 308 POST) with query
- nav: one prices entry replaces groceries and skins; cyan price-tag icon

src/routes/trackers/prices/groceries/+layout.server.ts

@@ -0,0 +1,10 @@
+import { requireOwner } from '#lib/server/owner.js';
+import type { LayoutServerLoad } from './$types';
+
+// Owner only, like tasks and health: everyone else gets a plain 404. Form actions skip
+// this load, so +page.server.ts checks again and hooks.server.ts blocks the prefix too.
+export const load: LayoutServerLoad = (event) => {
+	requireOwner(event);
+	event.setHeaders({ 'cache-control': 'private, no-store' });
+	return {};
+};