Derive device voltages from project settings

docs/circuit-list-editor-api.md

@@ -150,7 +150,11 @@ synchronizes linked circuit rows implicitly.
   - response:
     `{ "project": { ... }, "revision": { ... }, "history": { ... } }`
   - persistent Undo/Redo restores metadata, voltage values and the enabled
-    distribution-board supply types together
+    distribution-board supply types together; project-device and circuit
+    voltages are recalculated from the restored project settings in the same
+    transaction
+  - project-device and circuit voltage are derived values and are not accepted
+    as editable frontend fields
   - at least one of `AV`, `SV`, `EV`, `USV`, `MSR`, `SiBe` must be enabled;
     a supply type currently used by a distribution board cannot be disabled
   - unchanged values are rejected without creating a revision; a stale