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/TopBar.tsx

@@ -1,9 +1,9 @@
 import { useMemo, useRef, useState } from 'react'
-import { downloadProjectFile, readProjectFile, ProjectImportError } from '../../data/exportImport'
+import { downloadDiagramFile, readDiagramFile, DiagramImportError } from '../../data/exportImport'
 import { supabase } from '../../data/supabaseClient'
 import { useAdminReviewStore } from '../../state/adminReviewStore'
 import { useAuthStore } from '../../state/authStore'
-import { useProjectStore } from '../../state/projectStore'
+import { useDiagramStore } from '../../state/diagramStore'
 import { useSubmissionStore } from '../../state/submissionStore'
 import AdminUsersModal from '../admin/AdminUsersModal'
 import DiagramSharingModal from '../sharing/DiagramSharingModal'
@@ -33,11 +33,11 @@ function setLastSeenSubmissionsAt(iso: string): void {
 }
 
 export default function TopBar() {
-  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 canEdit = access?.myPermission !== 'view'
-  const renameProject = useProjectStore((s) => s.renameProject)
-  const importProject = useProjectStore((s) => s.importProject)
+  const renameDiagram = useDiagramStore((s) => s.renameDiagram)
+  const importDiagram = useDiagramStore((s) => s.importDiagram)
   const mySubmissions = useSubmissionStore((s) => s.mySubmissions)
   const username = useAuthStore((s) => s.username)
   const role = useAuthStore((s) => s.role)
@@ -66,7 +66,7 @@ export default function TopBar() {
     setLastSeenSubmissionsAt(new Date().toISOString())
   }
 
-  const handleExport = () => downloadProjectFile(project)
+  const handleExport = () => downloadDiagramFile(diagram)
 
   const handleImportClick = () => fileInputRef.current?.click()
 
@@ -75,10 +75,10 @@ export default function TopBar() {
     event.target.value = ''
     if (!file) return
     try {
-      const imported = await readProjectFile(file)
-      await importProject(imported)
+      const imported = await readDiagramFile(file)
+      await importDiagram(imported)
     } catch (err) {
-      const message = err instanceof ProjectImportError ? err.message : 'Could not read that file.'
+      const message = err instanceof DiagramImportError ? err.message : 'Could not read that file.'
       window.alert(`Import failed: ${message}`)
     }
   }
@@ -88,8 +88,8 @@ export default function TopBar() {
       <div className="flex items-center gap-2">
         <span className="text-sm font-semibold text-indigo-700">AV Planner</span>
         <input
-          value={project.name}
-          onChange={(e) => renameProject(e.target.value)}
+          value={diagram.name}
+          onChange={(e) => renameDiagram(e.target.value)}
           disabled={!canEdit}
           className="rounded border border-transparent px-2 py-1 text-sm text-slate-700 hover:border-slate-200 focus:border-slate-300 focus:outline-none disabled:cursor-not-allowed disabled:opacity-60"
         />