feat(demo): allow mutations in demo mode via DEMO_READ_ONLY flag (#6224)

.env.example

@@ -59,4 +59,11 @@ TURBO_TELEMETRY_DISABLED=1
 # ENABLE_KUBERNETES=true
 
 # Enable mock integration
-UNSAFE_ENABLE_MOCK_INTEGRATION=true
+UNSAFE_ENABLE_MOCK_INTEGRATION=true
+
+# Enable demo mode (seeds a demo user "demo"/"demo", a sample board and a mock integration)
+# DEMO_MODE=true
+
+# Whether demo mode is read-only. Defaults to true (the public demo blocks all mutations).
+# Set to false for writable preview deployments so the demo user can modify the dashboard.
+# DEMO_READ_ONLY=false