feat: enhance OIDC configuration for Azure login and improve error handling in API responses

Caddyfile

@@ -46,6 +46,13 @@
     }
 
     handle_errors {
+        # Don't mask API errors with the SPA shell — return a proper
+        # JSON error so the frontend can display a meaningful message.
+        @api path /api/*
+        respond @api `{"code":{err.status_code},"message":"Backend unavailable"}` {err.status_code} {
+            Content-Type application/json
+        }
+
         rewrite * /index.html
         file_server
     }