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.

README.md

@@ -18,13 +18,14 @@ src/
   domain/       Framework-agnostic types + logic (compatibility rules, BOM math,
                 the built-in connector/cable/device library). No React, no
                 React Flow — safe to unit test or reuse from a future backend.
-  data/         Storage abstraction. `ProjectRepository` is the interface the
-                rest of the app codes against; `LocalStorageProjectRepository`
-                is the only implementation today. Swapping in a real backend
-                later (REST/GraphQL) means adding one new implementation of
-                that interface, not touching state/UI code.
-  state/        Zustand store (`projectStore`) — the single source of truth for
-                the current project. Auto-saves to the repository (debounced)
+  data/         Storage abstraction. `DiagramRepository` is the interface the
+                rest of the app codes against; `SupabaseDiagramRepository` is
+                the active implementation, with `LocalStorageDiagramRepository`
+                kept as a reference/fallback. Swapping storage means adding
+                one new implementation of that interface, not touching
+                state/UI code.
+  state/        Zustand store (`diagramStore`) — the single source of truth for
+                the current diagram. Auto-saves to the repository (debounced)
                 on every change.
   components/
     canvas/     React Flow wiring surface: custom device node, custom cable
@@ -40,17 +41,19 @@ src/
 - **PortType** — a connector kind (HDMI, XLR, Cat5e/6, ...). Two ports can be
   wired together only if their PortTypes share a compatibility `family`.
 - **DeviceTemplate** — a reusable device shape (its ports), shown in the
-  palette. Built-ins live in `domain/library.ts`; custom ones are saved on the
-  project.
+  palette. Built-ins live in `domain/library.ts` and seed the public catalog;
+  custom ones are your own private catalog entries (or submitted for public
+  review).
 - **Device** — an instance placed on the canvas, with its own copy of ports
   (editing an instance never mutates its template).
 - **CableType** — a physical cable spec, scoped to a PortType family.
 - **Connection** — a wire between two ports, referencing a CableType and an
   optional user-entered distance (there's no floor plan / scale model, so
   length is a manual estimate per connection).
-- **Project** — devices + connections + any custom library entries. This is
-  the one object persisted (auto-saved to `localStorage`, or explicitly
-  exported/imported as JSON).
+- **Diagram** — devices + connections. This is the one object persisted
+  (auto-saved to Supabase, or explicitly exported/imported as JSON); the
+  catalog (PortType/CableType/DeviceTemplate/category) is shared and global,
+  not part of any one diagram.
 
 ## Development