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/lib/components/player/TransportControls.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 	import { player } from '$lib/player/player.svelte';
-	import PlayButton from '$lib/components/ui/PlayButton.svelte';
+	import PlayButton from '$lib/components/ui/media/PlayButton.svelte';
 	import Shuffle from '@lucide/svelte/icons/shuffle';
 	import Repeat from '@lucide/svelte/icons/repeat';