Rename Project to Diagram throughout the app

Per organized-ideas.md §8: the storage layer (DiagramRepository etc.) was
already renamed in an earlier phase; this finishes it everywhere else.

- domain/types.ts: Project -> Diagram. domain/project.ts -> domain/diagram.ts
  (createEmptyProject -> createEmptyDiagram, default name "Untitled Diagram").
- domain/compatibility.ts, domain/bom.ts: Project param/type -> Diagram.
- data/exportImport.ts: ProjectImportError -> DiagramImportError,
  projectToJson/downloadProjectFile/readProjectFile/normalizeProject -> their
  Diagram equivalents.
- state/projectStore.ts -> state/diagramStore.ts: useProjectStore ->
  useDiagramStore, the `project` field -> `diagram`, newProject/renameProject/
  importProject/applyRestoredProject -> *Diagram, restoredProjectUpdatedAt ->
  restoredDiagramUpdatedAt.
- Every component updated to match, compiler-guided (tsc -b enumerated each
  remaining call site after the core rename, the same approach used for the
  earlier catalog-parameter refactor).
- README updated for the terminology, and to match the repository class
  names (which had already been renamed but the README hadn't caught up).

No backend/schema changes: the JSON shape stored in diagrams.data never
changed, only TypeScript-side identifiers, so existing diagrams are
unaffected. One SQL comment fixed for accuracy (no migration needed).

Verified: tsc -b and oxlint clean; grepped src/ for any remaining
Project/project reference (none) after the sweep.

src/components/layout/DiagramManagerModal.tsx

@@ -1,5 +1,5 @@
 import { useEffect } from 'react'
-import { useProjectStore } from '../../state/projectStore'
+import { useDiagramStore } from '../../state/diagramStore'
 import Modal from '../common/Modal'
 
 function formatUpdatedAt(iso: string): string {
@@ -8,25 +8,25 @@ function formatUpdatedAt(iso: string): string {
 }
 
 export default function DiagramManagerModal({ onClose }: { onClose: () => void }) {
-  const project = useProjectStore((s) => s.project)
-  const diagramList = useProjectStore((s) => s.diagramList)
-  const refreshDiagramList = useProjectStore((s) => s.refreshDiagramList)
-  const newProject = useProjectStore((s) => s.newProject)
-  const switchToDiagram = useProjectStore((s) => s.switchToDiagram)
-  const deleteDiagram = useProjectStore((s) => s.deleteDiagram)
+  const diagram = useDiagramStore((s) => s.diagram)
+  const diagramList = useDiagramStore((s) => s.diagramList)
+  const refreshDiagramList = useDiagramStore((s) => s.refreshDiagramList)
+  const newDiagram = useDiagramStore((s) => s.newDiagram)
+  const switchToDiagram = useDiagramStore((s) => s.switchToDiagram)
+  const deleteDiagram = useDiagramStore((s) => s.deleteDiagram)
 
   useEffect(() => {
     refreshDiagramList()
   }, [refreshDiagramList])
 
   const handleOpen = async (id: string) => {
-    if (id === project.id) return
+    if (id === diagram.id) return
     await switchToDiagram(id)
     onClose()
   }
 
   const handleNew = async () => {
-    await newProject()
+    await newDiagram()
     onClose()
   }
 
@@ -52,7 +52,7 @@ export default function DiagramManagerModal({ onClose }: { onClose: () => void }
 
         <ul className="space-y-1">
           {diagramList.map((d) => {
-            const isCurrent = d.id === project.id
+            const isCurrent = d.id === diagram.id
             return (
               <li
                 key={d.id}