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/sharing/VersionHistoryModal.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useState } from 'react'
 import { useDiagramSnapshotStore } from '../../state/diagramSnapshotStore'
-import { useProjectStore } from '../../state/projectStore'
+import { useDiagramStore } from '../../state/diagramStore'
 import Modal from '../common/Modal'
 
 function formatDateTime(iso: string): string {
@@ -8,10 +8,10 @@ function formatDateTime(iso: string): string {
 }
 
 export default function VersionHistoryModal({ onClose }: { onClose: () => void }) {
-  const project = useProjectStore((s) => s.project)
-  const access = useProjectStore((s) => s.access)
+  const diagram = useDiagramStore((s) => s.diagram)
+  const access = useDiagramStore((s) => s.access)
   const canRestore = access?.myPermission === 'owner' || access?.myPermission === 'edit'
-  const diagramId = project.id
+  const diagramId = diagram.id
 
   const snapshots = useDiagramSnapshotStore((s) => s.snapshots)
   const isLoaded = useDiagramSnapshotStore((s) => s.isLoaded)
@@ -37,7 +37,7 @@ export default function VersionHistoryModal({ onClose }: { onClose: () => void }
   }
 
   return (
-    <Modal title={`Version History — "${project.name}"`} onClose={onClose} width="max-w-md">
+    <Modal title={`Version History — "${diagram.name}"`} onClose={onClose} width="max-w-md">
       <div className="space-y-1.5">
         <p className="text-[11px] text-slate-500">
           Checkpoints saved periodically while editing (up to the 50 most recent).