cfddns: auto-generate WEB_PASSWORD via random form-field type

The previous 'password' type required a manual value during app
update, which blocked Runtipi from regenerating the compose file
(error: 'Variable Web UI Password is required'). Switch to the
'random' field type with min=24 hex chars so a strong password is
auto-generated on install and persisted in app.env, viewable in
the dashboard.

apps/cfddns/config.json

@@ -30,12 +30,13 @@
       "hint": "Username for HTTP basic auth on the management UI"
     },
     {
-      "type": "password",
+      "type": "random",
       "label": "Web UI Password",
       "env_variable": "CFDDNS_WEB_PASSWORD",
       "required": true,
-      "min": 8,
-      "hint": "Password for HTTP basic auth on the management UI"
+      "min": 24,
+      "encoding": "hex",
+      "hint": "Auto-generated 24-char hex password for the UI. View it in the app's Settings tab and copy it to your browser when prompted."
     },
     {
       "type": "password",