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/lib/menus.svelte.ts

@@ -1,19 +1,18 @@
-import {
-	contextMenuStateFor,
-	type ContextMenuState
-} from '$lib/components/TrackContextMenu.svelte';
+import { contextMenuPosition, type MenuPosition } from '$lib/components/ui/ContextMenu.svelte';
 import { player } from './player/player.svelte';
 import { queueItemForTarget, type TrackTarget } from './tracks';
 
+export type TrackMenuState = TrackTarget & MenuPosition;
+
 export function createTrackMenu() {
-	let state = $state<ContextMenuState | null>(null);
+	let state = $state<TrackMenuState | null>(null);
 
 	return {
 		get state() {
 			return state;
 		},
 		open(event: MouseEvent, target: TrackTarget) {
-			state = contextMenuStateFor(event, target);
+			state = { ...target, ...contextMenuPosition(event) };
 		},
 		close() {
 			state = null;