Add Shlink Web Client install and update support

Add optional Shlink Web Client installation and update workflow, switch init to use shlink-installer, and expose the web UI on port 3000. Changes include: use vendor/bin/shlink-installer init (sourcing /opt/shlink/.env) instead of direct db:migrate/create; create data directories (cache, locks, logs, proxies, temp-geolite); add interactive prompt to install the web client, fetch and deploy shlink-web-client GitHub releases, generate /opt/shlink-web-client/servers.json with the initial API key, and configure an nginx site serving the client on port 3000. The ct update script now also checks for and updates the web client release. Update metadata (json/shlink.json) to set interface_port to 3000 and add an info note about API (8080) vs Web Client (3000). Also print the 3000 URL on completion.

json/shlink.json

@@ -8,7 +8,7 @@
     "type": "ct",
     "updateable": true,
     "privileged": false,
-    "interface_port": 8080,
+    "interface_port": 3000,
     "documentation": "https://shlink.io/documentation/",
     "website": "https://shlink.io/",
     "logo": "https://cdn.jsdelivr.net/gh/selfhst/icons@main/webp/shlink.webp",
@@ -39,6 +39,10 @@
         {
             "text": "Configure your short domain by editing DEFAULT_DOMAIN in /opt/shlink/.env and restarting the service.",
             "type": "info"
+        },
+        {
+            "text": "Shlink API runs on port 8080, the Web Client (if installed) on port 3000.",
+            "type": "info"
         }
     ]
 }