Switch mods to per-folder auto-discovery and resourcepack to single zip

- PackDefinition: replace mods[]/resourcepacks[] with modsFolder (string) + resourcepackPath (string); drop PackAsset
- Editor: replace dynamic add/remove lists with two single inputs; remove the now-dead JS for adding/removing rows
- Server: expose GET /file/mods/<folder>/index.json that returns the list of .jar names; folder name restricted to [a-zA-Z0-9_-]+
- Installer: fetch the listing JSON and download each jar from /file/mods/<folder>/<file>.jar; download the single resourcepack from /file/resourcepacks/<file>.zip directly into resourcepacks/

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

manifest/music-quiz.json

@@ -5,18 +5,8 @@
     "type": "forge",
     "downloadUrl": "/forge-installer.jar"
   },
-  "mods": [
-    {
-      "name": "ExampleMod",
-      "downloadUrl": "https://example.com/examplemod.jar"
-    }
-  ],
-  "resourcepacks": [
-    {
-      "name": "ExampleResourcePack",
-      "downloadUrl": "https://example.com/resourcepack.zip"
-    }
-  ],
+  "modsFolder": "music-quiz",
+  "resourcepackPath": "music-quiz.zip",
   "serverMinRam": 2048,
   "serverMaxRam": 8192,
   "clientMinRam": 4096,