Persist project settings updates

docs/circuit-list-editor-api.md

@@ -62,7 +62,8 @@ The public dispatcher currently supports `circuit.update`, `circuit.insert`,
 `circuit.reorder-section`, `circuit.reorder-sections`,
 `circuit.renumber-section` and
 `project-device.update`, `project-device.insert`, `project-device.delete` and
-`project-device.sync-rows`, all with schema version `1`. Other command types
+`project-device.sync-rows` as well as `project.update-settings`, all with
+schema version `1`. Other command types
 are rejected. Insert commands contain the complete entity or circuit block
 with stable ids; delete commands include the expected parent identity. Circuit
 snapshots contain zero, one or multiple complete device rows. Move commands
@@ -136,6 +137,18 @@ These write endpoints execute the typed `project-device.update`,
 returns `409 PROJECT_REVISION_CONFLICT`. Updating a project device still never
 synchronizes linked circuit rows implicitly.
 
+### Project Settings
+
+- `PUT /projects/:projectId`
+  - request:
+    `{ "expectedRevision": 12, "singlePhaseVoltageV": 230, "threePhaseVoltageV": 400 }`
+  - executes `project.update-settings` as one atomic revision
+  - response:
+    `{ "project": { ... }, "revision": { ... }, "history": { ... } }`
+  - persistent Undo/Redo restores both voltage values together
+  - unchanged values are rejected without creating a revision; a stale
+    revision returns `409 PROJECT_REVISION_CONFLICT`
+
 Example:
 
 ```json