refactor(mqttx): convert from LXC to addon

MQTTX Web is a static browser-based MQTT client with no backend.
Makes more sense as an addon installable into any existing container
than as a dedicated LXC.

Addon installs into /opt/mqttx, builds web/ with yarn, serves via
Nginx on configurable port (default 8095). Supports install/update/uninstall.

json/mqttx.json

@@ -1,40 +0,0 @@
-{
-    "name": "MQTTX Web",
-    "slug": "mqttx",
-    "categories": [
-        18
-    ],
-    "date_created": "2026-04-07",
-    "type": "ct",
-    "updateable": true,
-    "privileged": false,
-    "interface_port": 80,
-    "documentation": "https://mqttx.app/docs/web",
-    "website": "https://mqttx.app/",
-    "logo": "https://cdn.jsdelivr.net/gh/selfhst/icons@main/webp/mqttx.webp",
-    "config_path": "",
-    "description": "MQTTX Web is an open source MQTT 5.0 browser client tool. It uses WebSocket to connect to MQTT brokers directly from the browser, making it easy to test and debug MQTT services without local installation.",
-    "install_methods": [
-        {
-            "type": "default",
-            "script": "ct/mqttx.sh",
-            "resources": {
-                "cpu": 1,
-                "ram": 1024,
-                "hdd": 4,
-                "os": "Debian",
-                "version": "13"
-            }
-        }
-    ],
-    "default_credentials": {
-        "username": null,
-        "password": null
-    },
-    "notes": [
-        {
-            "text": "MQTTX Web connects to MQTT brokers via WebSocket directly from the browser. No credentials are required for the web UI itself.",
-            "type": "info"
-        }
-    ]
-}