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>

tests/building-structure-frontend-api.test.ts

@@ -0,0 +1,100 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  createBuilding,
+  createFloor,
+  deleteBuilding,
+  listBuildings,
+  updateBuilding,
+  updateFloor,
+} from "../src/frontend/utils/api.js";
+
+describe("building structure frontend API", () => {
+  it("sends building commands and optional floor building assignments", async () => {
+    const requests: Array<{ url: string; method: string; body: unknown }> = [];
+    const originalFetch = globalThis.fetch;
+    globalThis.fetch = async (input, init) => {
+      requests.push({
+        url: String(input),
+        method: init?.method ?? "GET",
+        body: init?.body === undefined ? undefined : JSON.parse(String(init.body)),
+      });
+      return new Response(JSON.stringify([]), {
+        status: 200,
+        headers: { "Content-Type": "application/json" },
+      });
+    };
+    try {
+      await listBuildings("project-1");
+      await createBuilding("project-1", { name: "Haus A" }, 3);
+      await updateBuilding("project-1", "building-1", { name: "Haus B" }, 4);
+      await deleteBuilding("project-1", "building-1", 5);
+      await createFloor(
+        "project-1",
+        { name: "EG", buildingId: "building-1" },
+        6
+      );
+      await createFloor("project-1", { name: "Dach", buildingId: null }, 7);
+      await createFloor("project-1", { name: "UG", buildingId: undefined }, 8);
+      await updateFloor(
+        "project-1",
+        "floor-1",
+        { name: "1. OG", buildingId: null },
+        9
+      );
+      await updateFloor("project-1", "floor-1", { name: "2. OG" }, 10);
+    } finally {
+      globalThis.fetch = originalFetch;
+    }
+
+    assert.deepEqual(requests, [
+      {
+        url: "/api/projects/project-1/buildings",
+        method: "GET",
+        body: undefined,
+      },
+      {
+        url: "/api/projects/project-1/buildings",
+        method: "POST",
+        body: { name: "Haus A", expectedRevision: 3 },
+      },
+      {
+        url: "/api/projects/project-1/buildings/building-1",
+        method: "PUT",
+        body: { name: "Haus B", expectedRevision: 4 },
+      },
+      {
+        url: "/api/projects/project-1/buildings/building-1",
+        method: "DELETE",
+        body: { expectedRevision: 5 },
+      },
+      {
+        url: "/api/projects/project-1/floors",
+        method: "POST",
+        body: { name: "EG", buildingId: "building-1", expectedRevision: 6 },
+      },
+      {
+        url: "/api/projects/project-1/floors",
+        method: "POST",
+        body: { name: "Dach", buildingId: null, expectedRevision: 7 },
+      },
+      {
+        url: "/api/projects/project-1/floors",
+        method: "POST",
+        body: { name: "UG", expectedRevision: 8 },
+      },
+      {
+        url: "/api/projects/project-1/floors/floor-1",
+        method: "PUT",
+        body: { name: "1. OG", buildingId: null, expectedRevision: 9 },
+      },
+      {
+        url: "/api/projects/project-1/floors/floor-1",
+        method: "PUT",
+        body: { name: "2. OG", expectedRevision: 10 },
+      },
+    ]);
+    const updateWithoutBuilding = requests[8].body as Record<string, unknown>;
+    assert.equal("buildingId" in updateWithoutBuilding, false);
+  });
+});