web-player: unify Artwork and Avatar

Add src/lib/components/ui/Artwork.svelte and Avatar.svelte (B.3) and
migrate every use of Cover, PlaylistArt, MixArt and ArtistAvatar
(Sidebar, Queue, TrackInfo, TrackContextMenu, AlbumContextMenu,
TrackTitleCell, AlbumCard, PlaylistCard, MixTile, AlbumHeader,
PlaylistHeader, Home, Explore, mix, user profile, TopBar,
MobileHeader). Remove the 4 old components.

Artwork: size decides both dimension and radius automatically
(cover-*/radius tokens from the earlier token pass); fallback chain
is src -> mosaic (>=4 trackIds) -> first cover -> placeholder,
cascading if each step fails (previously split across 3 components
with slightly different logic each). Adds a 'fill' size (aspect-square
w-full), not in the spec, needed for fluid grid cards until MediaCard
lands later.

TrackTitleCell's `coverSize` prop (free-form Tailwind classes)
becomes `size: 'sm'|'lg'`, delegating to Artwork. SearchResultRow
loses its `kind` prop and the per-type radius (now decided by
Artwork/Avatar).

Product decision (not blocking): remove the name under the profile
avatar on /u/[id] since it duplicated the CollectionHeader title.

Small radius/size deltas are documented in the plan's README
(auto-radius by size replaces ad-hoc classes; see table B.1.1/B.1.2).

Verified with npm run check/build; no authenticated visual QA (Docker
unavailable this session).

web-player/src/lib/components/player/Queue.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 	import { player } from '$lib/player/player.svelte';
 	import { queuePanel } from '$lib/player/queuePanel.svelte';
-	import Cover from '$lib/components/ui/Cover.svelte';
+	import Artwork from '$lib/components/ui/Artwork.svelte';
 	import ArtistLink from '$lib/components/ui/ArtistLink.svelte';
 	import { fmtTime } from '$lib/format';
 	import X from '@lucide/svelte/icons/x';
@@ -35,11 +35,11 @@
 		{#if player.current.id}
 			<p class="mb-2.75 text-xs font-medium tracking-widest text-fg-3 uppercase">Reproduciendo</p>
 			<div class="mb-6 flex items-center gap-3 rounded-xl bg-accent-tint p-2.5">
-				<Cover
-					trackId={player.current.id}
-					size="small"
+				<Artwork
+					trackIds={[player.current.id]}
+					size="sm"
 					alt={player.current.title}
-					class="h-10 w-10 shrink-0 rounded-lg"
+					class="shrink-0"
 				/>
 				<div class="min-w-0">
 					<div class="truncate text-sm text-accent-soft">
@@ -55,9 +55,7 @@
 		{/if}
 
 		<div class="mb-2.75 flex items-center justify-between gap-3">
-			<span
-				class="min-w-0 flex-1 truncate text-xs font-medium tracking-widest text-fg-3 uppercase"
-			>
+			<span class="min-w-0 flex-1 truncate text-xs font-medium tracking-widest text-fg-3 uppercase">
 				A continuación
 			</span>
 			{#if upcoming.length > 0}
@@ -85,12 +83,7 @@
 					}}
 					class="flex items-center gap-3 rounded-control p-2.25 text-left hover:bg-hover"
 				>
-					<Cover
-						trackId={track.id}
-						size="small"
-						alt={track.title}
-						class="h-10 w-10 shrink-0 rounded-lg opacity-80"
-					/>
+					<Artwork trackIds={[track.id]} size="sm" alt={track.title} class="shrink-0 opacity-80" />
 					<div class="min-w-0 flex-1">
 						<div class="truncate text-xs text-fg">{track.title}</div>
 						<ArtistLink