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>

docs/README.md

@@ -14,6 +14,8 @@ ausdrücklich getrennt und dürfen nicht als bereits implementiert verstanden we
   Drag-and-drop, Sortierung, Filterung und Undo/Redo
 - [Seitenansicht und PDF-Export](print-view.md) – Deckblatt,
   Verteilerübersicht, Verteilerpläne, Büro-Logo
+- [Gebäudestruktur](building-structure.md) – Gebäude → Etagen → Räume,
+  Befehle, Snapshot-Schema 7, eigene Seite
 - [Editor-API](circuit-list-editor-api.md) – unterstützte Circuit-First-Endpunkte
 - [Bekannte Einschränkungen](circuit-list-editor-known-limitations.md) –
   bewusst noch nicht implementierte oder begrenzte Funktionen