fix(desktop): point API base URL at api.healthchat.ch

The desktop variant config hardcoded ch.healthchat.api_base_url as
https://healthchat.ch, which is the wrong host. The fork's widget
auto-approval logic keys on the api.healthchat.ch origin, so the bare
domain bypassed it and pointed API traffic at the wrong host.

Stale since the desktop build pipeline was introduced (5f09f58a82).

apps/desktop/element.io/healthchat/config.json

@@ -99,7 +99,7 @@
     "update_base_url": "https://healthchat.ch/packages/desktop/update/",
     "modules": ["/modules/healthchat-module.js"],
     "ch.healthchat": {
-        "api_base_url": "https://healthchat.ch",
+        "api_base_url": "https://api.healthchat.ch",
         "elements_base_url": "/webapp/modules/elements"
     }
 }