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/app/globals.css

@@ -1461,3 +1461,144 @@ a.kpi:hover {
 .sidebar-collapsed .sidebar-label {
   display: none;
 }
+
+/* ── Gebäudestruktur (Gebäude → Etagen → Räume) ─────────────────────── */
+
+.structure-columns {
+  display: grid;
+  grid-template-columns: minmax(0, 1fr);
+  gap: var(--space-md);
+  align-items: start;
+}
+
+@media (min-width: 992px) {
+  .structure-columns {
+    grid-template-columns: repeat(3, minmax(0, 1fr));
+  }
+}
+
+.structure-column-title {
+  font-size: 1rem;
+  font-weight: 700;
+  margin: 0;
+  display: flex;
+  align-items: center;
+}
+
+.structure-icon-button {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 0.25rem 0.4rem;
+  line-height: 1;
+}
+
+.structure-back-link {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  padding: 0.35rem 0.45rem;
+  margin-top: 0.1rem;
+}
+
+.structure-list {
+  max-height: 70vh;
+  overflow-y: auto;
+}
+
+.structure-entry {
+  display: flex;
+  align-items: center;
+  gap: var(--space-sm);
+  padding: 0;
+}
+
+.structure-entry-main {
+  flex: 1 1 auto;
+  min-width: 0;
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: var(--space-sm);
+  padding: 0.55rem 0.75rem;
+  border: 0;
+  background: transparent;
+  color: inherit;
+  text-align: left;
+  font: inherit;
+}
+
+button.structure-entry-main {
+  cursor: pointer;
+}
+
+button.structure-entry-main:hover {
+  background: var(--surface-hover);
+}
+
+button.structure-entry-main:focus-visible {
+  outline: 2px solid var(--accent-blue);
+  outline-offset: -2px;
+}
+
+.structure-entry.is-selected {
+  background: var(--surface-selected);
+  box-shadow: inset 3px 0 0 var(--color-signal);
+}
+
+.structure-entry.is-selected .structure-entry-label {
+  font-weight: 600;
+}
+
+.structure-entry.is-selected button.structure-entry-main:hover {
+  background: transparent;
+}
+
+.structure-entry-text {
+  min-width: 0;
+  display: flex;
+  flex-direction: column;
+}
+
+.structure-entry-label {
+  overflow-wrap: anywhere;
+}
+
+.structure-entry-hint {
+  font-size: 0.78rem;
+  color: var(--color-ink-soft);
+}
+
+.structure-entry-meta {
+  flex: 0 0 auto;
+  font-size: 0.78rem;
+  color: var(--color-ink-soft);
+  white-space: nowrap;
+}
+
+.structure-entry-actions {
+  flex: 0 0 auto;
+  display: inline-flex;
+  gap: 4px;
+  padding-right: 0.75rem;
+}
+
+.structure-room-number {
+  font-weight: 600;
+  font-variant-numeric: tabular-nums;
+}
+
+.structure-group-label {
+  font-size: 0.66rem;
+  letter-spacing: 0.08em;
+  text-transform: uppercase;
+  font-weight: 600;
+  color: var(--color-accent);
+  background: var(--panel-bg-subtle);
+  padding: 0.35rem 0.75rem;
+}
+
+.structure-empty {
+  color: var(--color-ink-soft);
+  font-size: 0.9rem;
+}