Add CronMaster addon; remove legacy installers

Replace legacy container/install scripts with a single tools/addon/cronmaster.sh addon. The new script consolidates install, update and uninstall flows, uses community-scripts core functions, fetches prebuilt GitHub releases, sets up Node.js (defaults to v22), creates a systemd service and an update helper (/usr/local/bin/update_cronmaster). Removed ct/cronmaster.sh and install/cronmaster-install.sh to avoid duplication and centralize maintenance.

frontend/public/json/cronmaster.json

@@ -0,0 +1,35 @@
+{
+    "name": "CronMaster",
+    "slug": "cronmaster",
+    "categories": [
+        1
+    ],
+    "date_created": "2026-02-17",
+    "type": "pve",
+    "updateable": true,
+    "privileged": false,
+    "interface_port": 3000,
+    "documentation": "https://github.com/fccview/cronmaster",
+    "website": "https://github.com/fccview/cronmaster",
+    "logo": "https://raw.githubusercontent.com/fccview/cronmaster/main/public/logo.png",
+    "config_path": "/opt/cronmaster/.env",
+    "description": "Self-hosted cron job scheduler with web UI, live logs, auth and prebuilt binaries provided upstream.",
+    "install_methods": [
+        {
+            "type": "default",
+            "script": "tools/addon/cronmaster.sh",
+            "resources": {
+                "cpu": null,
+                "ram": null,
+                "hdd": null,
+                "os": null,
+                "version": null
+            }
+        }
+    ],
+    "default_credentials": {
+        "username": null,
+        "password": null
+    },
+    "notes": []
+}