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/db/repositories/building.repository.ts

@@ -0,0 +1,27 @@
+import { and, asc, eq } from "drizzle-orm";
+import type { AppDatabase } from "../database-context.js";
+import { buildings } from "../schema/buildings.js";
+
+export class BuildingRepository {
+  constructor(private readonly database: AppDatabase) {}
+
+  async listByProject(projectId: string) {
+    return this.database
+      .select()
+      .from(buildings)
+      .where(eq(buildings.projectId, projectId))
+      .orderBy(asc(buildings.sortOrder), asc(buildings.name));
+  }
+
+  async findById(projectId: string, buildingId: string) {
+    return (
+      this.database
+        .select()
+        .from(buildings)
+        .where(
+          and(eq(buildings.projectId, projectId), eq(buildings.id, buildingId))
+        )
+        .get() ?? null
+    );
+  }
+}