Migrate archinstall config to v4 format

archinstall v4.1 introduced new canonical key names. Update
user_configuration.json to the current schema:
- audio_config → nested under app_config
- bootloader → bootloader_config with explicit uki flag
- gfx_driver value updated to "All open-source (default)"
- custom-commands → custom_commands (underscore)

config/user_configuration.json

@@ -2,11 +2,16 @@
     "__comment": "ABOUTME: archinstall configuration for Moonarch.",
     "__comment2": "ABOUTME: Base setup — kernel, disk and filesystem are chosen interactively.",
 
-    "audio_config": {
-        "audio": "pipewire"
+    "app_config": {
+        "audio_config": {
+            "audio": "pipewire"
+        }
     },
 
-    "bootloader": "Systemd-boot",
+    "bootloader_config": {
+        "bootloader": "Systemd-boot",
+        "uki": false
+    },
 
     "kernels": ["linux-zen"],
 
@@ -52,7 +57,7 @@
     ],
 
     "profile_config": {
-        "gfx_driver": "All open-source",
+        "gfx_driver": "All open-source (default)",
         "greeter": "greetd",
         "profile": {
             "main": "Desktop",
@@ -71,7 +76,7 @@
 
     "timezone": "Europe/Berlin",
 
-    "custom-commands": [
+    "custom_commands": [
         "git clone https://gitea.moonarch.de/nevaforget/moonarch.git /opt/moonarch"
     ]
 }