Switch login to password-only and split pack zip paths

- Login form/route accepts password only; matched account row provides session userId
- PackDefinition: replace packPath with mapPath (.mc_custom/saves) and serverPath (server install dir); editor exposes two .zip fields
- Installer resolves relative platform/map/server URLs against manifest origin under /file/{platforms,maps,servers}/<name>; downloads and extracts the zips

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

manifest/music-quiz.json

@@ -3,7 +3,7 @@
   "mcVersion": "1.20.1",
   "platform": {
     "type": "forge",
-    "downloadUrl": "https://example.com/forge-installer.jar"
+    "downloadUrl": "/forge-installer.jar"
   },
   "mods": [
     {
@@ -21,5 +21,6 @@
   "serverMaxRam": 8192,
   "clientMinRam": 4096,
   "clientRecommendedRam": 8192,
-  "packPath": "music-quiz/files"
+  "mapPath": "music-quiz-map.zip",
+  "serverPath": "music-quiz-server.zip"
 }