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/MobileHeader.svelte

@@ -5,6 +5,7 @@
 	import LogIn from '@lucide/svelte/icons/log-in';
 	import MenuItem from './ui/MenuItem.svelte';
 	import AccountMenu from './ui/AccountMenu.svelte';
+	import Avatar from './ui/Avatar.svelte';
 
 	interface Props {
 		user: SessionUser | null;
@@ -40,15 +41,7 @@
 						aria-expanded={open}
 						class="flex items-center rounded-full"
 					>
-						{#if user.picture}
-							<img src={user.picture} alt="" class="h-9 w-9 rounded-full object-cover" />
-						{:else}
-							<span
-								class="grid h-9 w-9 place-items-center rounded-full bg-surface-2 text-sm uppercase"
-							>
-								{user.name.charAt(0)}
-							</span>
-						{/if}
+						<Avatar name={user.name} src={user.picture} size="sm" />
 					</button>
 				{/snippet}
 				{#snippet extraItems({ close })}