feat: store .jsdos bundle inside game directory instead of as flat file

Before: data/games/{id}.jsdos (flat, alongside the game dir)
After:  data/games/{id}/{id}.jsdos (inside the game directory)

Benefits:
- Delete is atomic — removing the game dir removes everything
- Cleaner data directory structure
- No orphan .jsdos files on delete

Also updated frontend bundleUrl() to use game.bundle_file from
the backend, making it path-agnostic. Old flat layout is still
served via StaticResource for backward compat.

frontend/src/lib/api.js

@@ -106,7 +106,9 @@ export function artworkUrl(path) {
 /** Get game bundle URL for normal play */
 export function bundleUrl(game) {
   if (!game) return null;
-  return `/games/${game.id}.jsdos`;
+  // Use the bundle_file path from the backend (handles both old flat layout
+  // and new in-directory layout transparently)
+  return `/games/${game.bundle_file}`;
 }
 
 /** Get game bundle URL for setup mode (runs SETUP.EXE instead) */