Add buildings above floors and a building structure page

Projects with several buildings could not be structured: floors and rooms
hung directly off the project. Floors can now belong to a building
(migration 0009: table buildings, nullable floors.building_id). Rooms and
distribution boards keep their floor; their building follows from it.
Existing floors stay without a building.

Buildings use the location-structure command store with
project-building.insert/.update/.delete, inverses and persistent Undo/Redo;
deleting a building with assigned floors is rejected. Floor commands carry
buildingId since command version 2. Version 1 floor commands from existing
history predate buildings and are upgraded with buildingId = null before
they are executed and persisted, so forward and inverse keep one version.

Project snapshot schema 7 stores buildings and floor.buildingId; versions
1-6 are upgraded without buildings. Restore and portable transfer include
buildings, and the transfer remaps building ids.

Buildings, floors and rooms move from the project overview to their own
page /projects/:id/struktur with three columns (Gebäude -> Etagen -> Räume)
that filter each other, and a sidebar entry. Floor selects in the board and
room dialogs are grouped by building. Unchanged dialogs close without a
command. Details in docs/building-structure.md; see issue #6.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/frontend/utils/api.ts

@@ -1,5 +1,7 @@
 import type {
+  BuildingDto,
   CircuitListDto,
+  CreateBuildingInput,
   CreateFloorInput,
   CreateProjectDeviceInput,
   CreateRoomInput,
@@ -17,6 +19,7 @@ import type {
   ProjectCommandResultDto,
   ProjectDeviceCommandResultDto,
   ProjectDeviceDto,
+  ProjectBuildingCommandResultDto,
   ProjectFloorCommandResultDto,
   ProjectHistoryStateDto,
   ProjectRevisionPageDto,
@@ -881,6 +884,61 @@ export function renumberCircuitSectionCommand(
   );
 }
 
+export function listBuildings(projectId: string) {
+  return request<BuildingDto[]>(`/api/projects/${projectId}/buildings`);
+}
+
+export function createBuilding(
+  projectId: string,
+  input: CreateBuildingInput,
+  expectedRevision: number
+) {
+  return request<ProjectBuildingCommandResultDto>(
+    `/api/projects/${projectId}/buildings`,
+    {
+      method: "POST",
+      body: JSON.stringify({ name: input.name, expectedRevision }),
+    }
+  );
+}
+
+export function updateBuilding(
+  projectId: string,
+  buildingId: string,
+  input: CreateBuildingInput,
+  expectedRevision: number
+) {
+  return request<ProjectBuildingCommandResultDto>(
+    `/api/projects/${projectId}/buildings/${buildingId}`,
+    {
+      method: "PUT",
+      body: JSON.stringify({ name: input.name, expectedRevision }),
+    }
+  );
+}
+
+export function deleteBuilding(
+  projectId: string,
+  buildingId: string,
+  expectedRevision: number
+) {
+  return request<ProjectCommandResultDto>(
+    `/api/projects/${projectId}/buildings/${buildingId}`,
+    {
+      method: "DELETE",
+      body: JSON.stringify({ expectedRevision }),
+    }
+  );
+}
+
+// buildingId is only sent when the caller passes it (string or null);
+// an omitted buildingId keeps the current assignment on update.
+function toFloorRequestBody(input: CreateFloorInput, expectedRevision: number) {
+  return input.buildingId === undefined
+    ? { name: input.name, expectedRevision }
+    : { name: input.name, buildingId: input.buildingId, expectedRevision };
+}
+
 export function listFloors(projectId: string) {
   return request<FloorDto[]>(`/api/projects/${projectId}/floors`);
 }
@@ -894,7 +952,7 @@ export function createFloor(
     `/api/projects/${projectId}/floors`,
     {
       method: "POST",
-      body: JSON.stringify({ ...input, expectedRevision }),
+      body: JSON.stringify(toFloorRequestBody(input, expectedRevision)),
     }
   );
 }
@@ -907,7 +965,7 @@ export function updateFloor(
 ) {
   return request<ProjectFloorCommandResultDto>(`/api/projects/${projectId}/floors/${floorId}`, {
     method: "PUT",
-    body: JSON.stringify({ ...input, expectedRevision }),
+    body: JSON.stringify(toFloorRequestBody(input, expectedRevision)),
   });
 }