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/components/Sidebar.tsx

@@ -7,14 +7,21 @@ import { listProjects } from "../utils/api";
 import type { ProjectDto } from "../types";
 import { ThemeToggle } from "./ThemeToggle";
 
+// Anchors on the project overview page (/projects/[id]).
 const PROJECT_SECTIONS = [
   { id: "verlauf", label: "Verlauf", icon: "↺" },
   { id: "verteilungen", label: "Verteilungen", icon: "⌁" },
-  { id: "etagen", label: "Etagen", icon: "▥" },
-  { id: "raeume", label: "Räume", icon: "⌂" },
   { id: "projektgeraete", label: "Projektgeräte", icon: "⚙" },
 ] as const;
 
+// Separate project sub-routes (/projects/[id]/<segment>).
+const PROJECT_ROUTES = [
+  { segment: "struktur", label: "Gebäudestruktur", icon: "▥" },
+] as const;
+
+// Matches the project overview and all of its sub-routes (group 2 = rest).
+const PROJECT_PATH_PATTERN = /^\/projects\/([^/]+)(\/.*)?$/;
+
 interface SidebarProps {
   isCollapsed: boolean;
   onToggleCollapsed: () => void;
@@ -22,8 +29,14 @@ interface SidebarProps {
 
 export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
   const pathname = usePathname();
-  const projectMatch = pathname?.match(/^\/projects\/([^/]+)$/);
+  const projectMatch = pathname?.match(PROJECT_PATH_PATTERN);
   const activeProjectId = projectMatch ? projectMatch[1] : null;
+  const projectSubPath = projectMatch?.[2] ?? "";
+  // Section anchors only exist (and are observed) on the overview itself.
+  const overviewProjectId =
+    activeProjectId && (projectSubPath === "" || projectSubPath === "/")
+      ? activeProjectId
+      : null;
 
   const [projects, setProjects] = useState<ProjectDto[]>([]);
   const [activeSection, setActiveSection] = useState<string | null>(null);
@@ -42,7 +55,7 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
   }, []);
 
   useEffect(() => {
-    if (!activeProjectId) {
+    if (!overviewProjectId) {
       setActiveSection(null);
       return;
     }
@@ -69,7 +82,7 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
 
     elements.forEach((element) => observer.observe(element));
     return () => observer.disconnect();
-  }, [activeProjectId]);
+  }, [overviewProjectId]);
 
   return (
     <aside className={isCollapsed ? "sidebar sidebar-collapsed" : "sidebar"}>
@@ -122,17 +135,46 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
         {activeProjectId && (
           <>
             <div className="sidebar-section-label">In diesem Projekt</div>
-            {PROJECT_SECTIONS.map(({ id, label, icon }) => (
-              <a
-                key={id}
-                href={`#${id}`}
-                className={activeSection === id ? "sidebar-link active" : "sidebar-link"}
-                title={label}
-              >
-                <span className="sidebar-icon">{icon}</span>
-                <span className="sidebar-label">{label}</span>
-              </a>
-            ))}
+            {PROJECT_SECTIONS.map(({ id, label, icon }) =>
+              overviewProjectId ? (
+                <a
+                  key={id}
+                  href={`#${id}`}
+                  className={activeSection === id ? "sidebar-link active" : "sidebar-link"}
+                  title={label}
+                >
+                  <span className="sidebar-icon">{icon}</span>
+                  <span className="sidebar-label">{label}</span>
+                </a>
+              ) : (
+                <Link
+                  key={id}
+                  href={`/projects/${activeProjectId}#${id}`}
+                  className="sidebar-link"
+                  title={label}
+                >
+                  <span className="sidebar-icon">{icon}</span>
+                  <span className="sidebar-label">{label}</span>
+                </Link>
+              )
+            )}
+            {PROJECT_ROUTES.map(({ segment, label, icon }) => {
+              const href = `/projects/${activeProjectId}/${segment}`;
+              const isActive =
+                pathname === href || Boolean(pathname?.startsWith(`${href}/`));
+              return (
+                <Link
+                  key={segment}
+                  href={href}
+                  aria-current={isActive ? "page" : undefined}
+                  className={isActive ? "sidebar-link active" : "sidebar-link"}
+                  title={label}
+                >
+                  <span className="sidebar-icon">{icon}</span>
+                  <span className="sidebar-label">{label}</span>
+                </Link>
+              );
+            })}
           </>
         )}
       </nav>