refactor: drop dead code left after the worktree move

ChipGroup is orphaned since the explore rewrite dropped the filter chips.
Mix.GeneratedOn was written on every generation and never read, and it
duplicated CreatedAt, which IAuditable already sets at the same moment.

web-player/src/lib/api/schema.d.ts

@@ -608,8 +608,6 @@ export interface components {
             id: string;
             title: string;
             subtitle: null | string;
-            /** Format: date */
-            generatedOn: string;
             /** Format: int32 */
             itemCount: number | string;
             items: components["schemas"]["MixItemApplicationResponse"][];

web-player/src/lib/components/ui/ChipGroup.svelte

@@ -1,57 +0,0 @@
-<script lang="ts" module>
-	import type { LucideIcon } from '@lucide/svelte';
-
-	export interface ChipOption {
-		value: string;
-		label: string;
-		icon?: LucideIcon;
-		disabled?: boolean;
-		hint?: string;
-	}
-</script>
-
-<script lang="ts">
-	import Check from '@lucide/svelte/icons/check';
-
-	interface Props {
-		options: ChipOption[];
-		selected: string[];
-		label: string;
-		onChange: (selected: string[]) => void;
-		class?: string;
-	}
-
-	let { options, selected, label, onChange, class: klass = '' }: Props = $props();
-
-	function toggle(value: string) {
-		onChange(selected.includes(value) ? selected.filter((v) => v !== value) : [...selected, value]);
-	}
-</script>
-
-<div role="group" aria-label={label} class="flex flex-wrap items-center gap-2 {klass}">
-	{#each options as option (option.value)}
-		{@const active = selected.includes(option.value)}
-		<label
-			title={option.hint}
-			class="inline-flex cursor-pointer items-center gap-1.5 rounded-full border px-3.5 py-1.5 text-sm font-semibold transition select-none
-				{active
-				? 'border-transparent bg-accent text-on-accent'
-				: 'border-line bg-surface text-fg-2 hover:bg-surface-hover'}
-				{option.disabled ? 'pointer-events-none opacity-45' : ''}"
-		>
-			<input
-				type="checkbox"
-				class="sr-only"
-				checked={active}
-				disabled={option.disabled}
-				onchange={() => toggle(option.value)}
-			/>
-			{#if active}
-				<Check class="h-3.5 w-3.5" strokeWidth={3} />
-			{:else if option.icon}
-				<option.icon class="h-3.5 w-3.5" strokeWidth={2} />
-			{/if}
-			{option.label}
-		</label>
-	{/each}
-</div>