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/state/catalogStore.ts

@@ -10,7 +10,7 @@ const repository: CatalogRepository = new SupabaseCatalogRepository()
 
 // Which public device templates you've hidden from your own palette — a
 // per-user UI preference, not catalog data, so it's not a Supabase row: hiding
-// a public entry only affects your view of it (see domain/project.ts's
+// a public entry only affects your view of it (see domain/diagram.ts's
 // allDeviceTemplates/hiddenDeviceTemplates), never the shared entry itself.
 const HIDDEN_PUBLIC_TEMPLATES_KEY = 'av-planner:hiddenPublicDeviceTemplateIds'
 function loadHiddenPublicIds(): Set<string> {