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/+layout.svelte

@@ -0,0 +1,14 @@
+<script lang="ts">
+	// One price tracker, a tab per thing (#lib/price-trackers). Each tab keeps its own page,
+	// loader and data; this only adds the tab bar. Private tabs are only listed for me.
+	import { page } from '$app/state';
+	import Tag from 'lucide-svelte/icons/tag';
+	import SpottingSwitcher from '#lib/components/SpottingSwitcher.svelte';
+	import { visiblePriceTabs } from '#lib/price-trackers.js';
+
+	let { children } = $props();
+	const tabs = $derived(visiblePriceTabs(Boolean(page.data.isOwner)));
+</script>
+
+<SpottingSwitcher items={tabs} home="/trackers/prices" label="prices" homeIcon={Tag} />
+{@render children()}