web-player: unify the context menu (ContextMenu) + AccountMenu

Add src/lib/components/ui/ContextMenu.svelte, replacing
TrackContextMenu and AlbumContextMenu (109 and 119 lines, 90%
identical): same positioning function (contextMenuPosition, exported
from the module, using MENU_WIDTH/SUBMENU_WIDTH moved to config.ts),
same backdrop, same Escape handling, same generic "Add to playlist"
submenu via {items, playlistAction, playlists}. Uses MenuItem for
actions (previously each reimplemented its own classes by hand) and
Artwork size="xs" for playlist thumbnails.

menus.svelte.ts (createTrackMenu) adapts to the new
contextMenuPosition; Home, Explore (track and album menus) and Mix
are migrated.

AccountMenu: panelClass (free-form Tailwind classes) becomes prop
width: 'sm'|'md'; adds Escape-to-close (accessibility improvement, it
didn't close before); TopBar and MobileHeader no longer wrap it in a
duplicate div.relative (its own root already is one). The panel's
z-index becomes z-(--z-menu), matching ContextMenu (previously z-20
in TopBar / no explicit z in MobileHeader).

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

web-player/src/routes/+page.svelte

@@ -27,7 +27,8 @@
 	import SectionHeading from '$lib/components/ui/SectionHeading.svelte';
 	import EmptyState from '$lib/components/ui/EmptyState.svelte';
 	import Button from '$lib/components/ui/Button.svelte';
-	import TrackContextMenu from '$lib/components/TrackContextMenu.svelte';
+	import ContextMenu from '$lib/components/ui/ContextMenu.svelte';
+	import ListPlus from '@lucide/svelte/icons/list-plus';
 	import { createTrackMenu } from '$lib/menus.svelte';
 	import { HOME_CONTINUE_LIMIT, HOME_POPULAR_MAX } from '$lib/config';
 
@@ -423,10 +424,19 @@
 </div>
 
 {#if trackMenu.state}
-	<TrackContextMenu
-		menu={trackMenu.state}
-		{playlists}
+	<ContextMenu
+		x={trackMenu.state.x}
+		y={trackMenu.state.y}
+		openLeft={trackMenu.state.openLeft}
 		onClose={() => trackMenu.close()}
-		onAddToQueue={() => trackMenu.playNext()}
+		items={[
+			{ icon: ListPlus, label: 'Reproducir a continuación', onclick: () => trackMenu.playNext() }
+		]}
+		playlistAction={{
+			action: '?/addTrack',
+			fields: { trackId: String(trackMenu.state.track.id) },
+			label: 'Añadir a una playlist'
+		}}
+		{playlists}
 	/>
 {/if}