add series grouping, cover upload, and multi-zip download to games library

- Series grouping: a top-level folder with no .zip but game subfolders is
  now treated as a GameSeries. Clicking a series drills into it with a
  breadcrumb; a game-count badge distinguishes series cards from game cards.
  Series fall back to the first game's cover when no series-level cover exists.
- Cover upload: new POST /api/game-cover endpoint writes cover.jpg or
  widecover.jpg directly into the game/series folder (re-encoded via sharp).
  A kebab menu on GameDetailModal opens an Edit Images panel showing previews
  and upload/replace buttons for both cover and wide cover.
- Multi-zip download: Game.zipFiles replaces zipPath and includes all .zip
  files in the folder. A single zip shows the existing download button; multiple
  zips render a split button — primary action downloads the first file, a
  dropdown arrow lists all files by name.

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

src/types/index.ts

@@ -13,7 +13,15 @@ export interface Game {
   title: string
   coverUrl: string | null
   wideCoverUrl: string | null
-  zipPath: string
+  zipFiles: string[]
+}
+
+export interface GameSeries {
+  id: string
+  title: string
+  coverUrl: string | null
+  wideCoverUrl: string | null
+  games: Game[]
 }
 
 export type MediaType = 'video' | 'image' | 'other'