web-player: buttons (Button, IconButton, PlayButton)

Button: subtle variants (primary/secondary, left unchanged per the
user's call), glass (formerly secondaryOnGlow, with its own
font-normal), accent, strong (cta-strong/ink, replacing the `!`
overrides on the Home hero), danger. Uses the --color-btn* tokens
instead of bg-[var(--mf-btn-...)]. Radius unified to rounded-control.

IconButton: adds shape (square/round), size (xs/sm/md/lg), tone
(muted/subtle/plain), pressed (aria-pressed + text-accent),
href/reload (links), surface (permanent background + hover to
surface-hover instead of the generic `hover` alias, which matched
surface-2 and gave no contrast).

New PlayButton: a single place for the play/pause SVGs, strong/glass
variants, as="span" for non-interactive overlays. Replaces
TransportControls' main button, the MixTile overlay, and the "Top
result" circle in Explore.

Migrated to IconButton: close queue, "Create playlist" (Sidebar),
Like (TrackInfo, with pressed), queue (PlayerExtras), back (TopBar),
login (MobileHeader), and the row actions in tables (remove/add, size
xs). BackLink uses size-icon-sm and drops a duplicate hover.

Decision (not blocking): remove TopBar's decorative "forward" button
and PlayerExtras' decorative AlignJustify icon (visually dead code).
TransportControls' buttons (shuffle/prev/next/repeat) are
deliberately left unmigrated: they currently have no background or
hit-box of their own, and IconButton always applies a fixed-size
box — migrating them would have added a visible hover pill that
couldn't be verified without Docker this session.

Verified with npm run check/build.

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

@@ -5,6 +5,7 @@
 	import { createPlaylistModal } from '$lib/playlists.svelte';
 	import type { SessionUser } from '$lib/types';
 	import Artwork from './ui/Artwork.svelte';
+	import IconButton from './ui/IconButton.svelte';
 	import Home from '@lucide/svelte/icons/house';
 	import Compass from '@lucide/svelte/icons/compass';
 	import ListMusic from '@lucide/svelte/icons/list-music';
@@ -77,14 +78,14 @@
 		<div class="flex items-center justify-between px-3 pb-1.5">
 			<span class="text-xs font-medium tracking-widest text-muted uppercase">Tus playlists</span>
 			{#if playlists.length > 0}
-				<button
-					type="button"
+				<IconButton
+					label="Crear playlist"
+					tone="plain"
+					size="xs"
 					onclick={() => createPlaylistModal.show()}
-					aria-label="Crear playlist"
-					class="text-muted transition-colors hover:text-fg"
 				>
-					<Plus class="h-3.5 w-3.5" strokeWidth={1.25} />
-				</button>
+					<Plus class="size-icon-sm" strokeWidth={1.25} />
+				</IconButton>
 			{/if}
 		</div>