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.

src/main/java/com/dostalgia/GameService.java

@@ -127,9 +127,8 @@ public class GameService {
                 }
             });
         }
-        // Remove flat .jsdos bundle
+        // Clean up old flat .jsdos locations (pre-game-dir layout — backward compat)
         Files.deleteIfExists(gamesDir.resolve(id + ".jsdos"));
-        // Remove flat setup .jsdos bundle (if any)
         Files.deleteIfExists(gamesDir.resolve(id + ".setup.jsdos"));
         return true;
     }

src/main/java/com/dostalgia/UploadResource.java

@@ -95,9 +95,9 @@ public class UploadResource {
             Path gameDir = svc.gameDir(gameId);
             Files.createDirectories(gameDir);
 
-            // Create .jsdos bundle (ZIP with game files + .jsdos/ config)
-            String bundleFile = gameId + ".jsdos";
-            Path bundlePath = svc.getGamesDir().resolve(bundleFile);
+            // Create .jsdos bundle inside the game directory
+            String bundleFile = gameId + "/" + gameId + ".jsdos";
+            Path bundlePath = gameDir.resolve(gameId + ".jsdos");
             createBundle(extractDir, mainExe, bundlePath);
 
             // Detect platform (DOS vs Windows) by reading the main EXE header