feat: initial app template catalog with 16 apps

Populate the appstore-apptemplates repo with catalog.json and
per-app config files for all 16 curated Docker apps, each at v1.0.0.

apps/nextcloud/app.json

@@ -0,0 +1,10 @@
+{
+  "id": "nextcloud",
+  "name": "Nextcloud",
+  "description": "Self-hosted productivity and file sharing platform",
+  "category": "Cloud Storage",
+  "iconName": "nextcloud",
+  "latestVersion": "1.0.0",
+  "versions": ["1.0.0"],
+  "maintainer": "serve.zone"
+}