Categorize components/ui into primitives, overlay, media, forms and layout

Splits the flat 38-component ui/ folder into primitives/ (generic
controls), overlay/ (menus/modals), media/ (music-domain display
components), forms/ (composite entity forms) and layout/ (page
scaffolding), and updates every import path accordingly.

web-player/src/routes/+error.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { page } from '$app/state';
-	import Button from '$lib/components/ui/Button.svelte';
+	import Button from '$lib/components/ui/primitives/Button.svelte';
 </script>
 
 <div class="animate-enter mx-auto my-[14vh] max-w-prose-sm text-center">