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/project-version-history.ts

@@ -46,6 +46,9 @@ const commandTypeLabels: Record<string, string> = {
   "circuit.move-group": "Stromkreis in andere Gruppe verschoben",
   "circuit-group.delete-subtree": "Stromkreisgruppe vollständig entfernt",
   "circuit-group.restore-subtree": "Stromkreisgruppe vollständig wiederhergestellt",
+  "project-building.insert": "Gebäude angelegt",
+  "project-building.update": "Gebäude bearbeitet",
+  "project-building.delete": "Gebäude entfernt",
   "project-floor.insert": "Geschoss angelegt",
   "project-floor.update": "Geschoss bearbeitet",
   "project-floor.delete": "Geschoss entfernt",