feat(most-played): add dedicated Most Played page (#86)

New sidebar item (TrendingUp icon, /most-played route) with:

- Top Artists section: derived by grouping frequent albums by artistId
  and summing playCount — no extra API calls needed. Shows up to 10
  artists as a responsive grid with avatar, rank and total play count.
- Top Albums section: paginated list using getAlbumList('frequent'),
  50 albums per batch with a "Load more" button. Each row shows rank,
  cover thumbnail (44px), album name, artist (navigates to artist
  page), year and play count.
- Sort toggle (most plays first / fewest plays first).

Also adds playCount?: number to SubsonicAlbum — Navidrome returns this
field in getAlbumList2?type=frequent but it was missing from the type.

Closes #86

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/store/sidebarStore.ts

@@ -18,6 +18,7 @@ export const DEFAULT_SIDEBAR_ITEMS: SidebarItemConfig[] = [
   { id: 'randomMix',     visible: true },
   { id: 'favorites',     visible: true },
   { id: 'playlists',     visible: true },
+  { id: 'mostPlayed',    visible: true },
   { id: 'radio',         visible: true },
   { id: 'statistics',    visible: true },
   { id: 'help',          visible: true },