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/domain/compatibility.ts

@@ -1,5 +1,5 @@
-import { acceptedFamilies, getPortType, hasBridgingCableType } from './project'
-import type { Catalog, Connection, Device, PortType, Project } from './types'
+import { acceptedFamilies, getPortType, hasBridgingCableType } from './diagram'
+import type { Catalog, Connection, Device, Diagram, PortType } from './types'
 
 /**
  * Two ports connect if the sets of families they each accept (their own
@@ -36,10 +36,10 @@ export interface ConnectionValidation {
  * server-side later since it only depends on plain domain data.
  */
 export function validateConnection(
-  project: Project,
+  diagram: Diagram,
   catalog: Catalog,
   candidate: ConnectionCandidate,
-  existingConnections: Connection[] = project.connections,
+  existingConnections: Connection[] = diagram.connections,
 ): ConnectionValidation {
   const { sourceDeviceId, sourcePortId, targetDeviceId, targetPortId } = candidate
 
@@ -47,8 +47,8 @@ export function validateConnection(
     return { valid: false, reason: 'A port cannot connect to itself.' }
   }
 
-  const sourceDevice = project.devices.find((d) => d.id === sourceDeviceId)
-  const targetDevice = project.devices.find((d) => d.id === targetDeviceId)
+  const sourceDevice = diagram.devices.find((d) => d.id === sourceDeviceId)
+  const targetDevice = diagram.devices.find((d) => d.id === targetDeviceId)
   const sourcePort = sourceDevice?.ports.find((p) => p.id === sourcePortId)
   const targetPort = targetDevice?.ports.find((p) => p.id === targetPortId)