Enable Zustand (proposal #5) + convert isCreatingRoom singleton

Phase 2 of the refactor plan in docs/ARCHITECTURE.md.

Install
- yarn add zustand (^5, matches React 19 peer requirement).

Wiring
- src/state/createNitroStore.ts: replaces the previous prototype
  (which threw on call) with a re-export of zustand's `create` under
  the project-local name `createNitroStore`. Comments document the
  convention (one store per domain, subscribe to slices not the whole
  store).

First migration target
- src/components/navigator/views/navigatorRoomCreatorStore.ts (new):
  a Zustand store with `isCreating: boolean` and `beginCreate()` —
  the latter latches the flag to true, dispatches an internal
  setTimeout to auto-reset after 5s, and replaces any in-flight timer
  on re-entry. The timer handle lives in the store's closure, so a
  remount of the view doesn't reset the lockout and StrictMode's
  double-mount no longer schedules two pending timers.
- src/components/navigator/views/NavigatorRoomCreatorView.tsx:
  removes the two module-level `let` variables that the React Compiler
  was flagging ("Writing to a variable defined outside a component is
  not allowed"). The component now reads `isCreating` via a slice
  subscription and calls `beginCreate()` from the click handler. The
  imperative guard (`if (isCreating) return`) uses
  `useRoomCreatorStore.getState()` so it reads the latest value
  synchronously without being a stale closure.
- Also cleans up `FC<{}>` -> `FC` while touching the file.

Verification
- yarn eslint on the three touched files: 1 pre-existing error
  (the `setCategory(categories[0].id)` set-state-in-effect on the
  categories hook, deliberately left as-is in Phase C — it's the
  "init from late-arriving async data" pattern; baseline matches).
- yarn tsc: clean.

Migration path (per docs/ARCHITECTURE.md)
- This is the smallest possible Zustand pilot (~30 lines), chosen
  because the let-singleton anti-pattern was the most obvious quick
  win and the React Compiler was already complaining about it.
- Next adoption targets (cross-feature UI state): the toolbar's
  active-window state (currently inside scattered Contexts), the
  notification center's open-state, the catalog's currentPage/selection
  state (after the god-hook split).

https://claude.ai/code/session_01GrR87LAqnAEyKG2ZbmQt5Q

src/state/createNitroStore.ts

@@ -1,42 +1,15 @@
 /**
- * Skeleton for proposal #5 (unified UI store).
+ * Re-export of zustand's `create` under a project-local name.
  *
- * NOT YET ENABLED — `zustand` is not in package.json.
- * To activate:
+ * Convention: each domain owns one store file. Either:
+ *   - `src/state/<domain>.ts` for cross-feature stores
+ *   - `src/components/<area>/<feature>Store.ts` for feature-local stores
  *
- *   yarn add zustand
+ * Components subscribe to specific slices only:
  *
- * Then this file becomes:
+ *   const isCreating = useNavigatorRoomCreatorStore(s => s.isCreating);
  *
- *   import { create } from 'zustand';
- *   export const createNitroStore = create;
- *
- * The naming convention below documents the intended structure: each
- * feature owns one slice file under `src/features/<feature>/state/`,
- * importing `createNitroStore` from here.
- *
- * Example slice (to be created when zustand is installed):
- *
- *   // src/features/wired-tools/state/wiredToolsSlice.ts
- *   import { createNitroStore } from '../../../state/createNitroStore';
- *
- *   type WiredToolsState = {
- *       activeTab: 'monitor' | 'variables' | 'inspection' | 'chests' | 'settings';
- *       setActiveTab: (tab: WiredToolsState['activeTab']) => void;
- *   };
- *
- *   export const useWiredToolsStore = createNitroStore<WiredToolsState>()((set) => ({
- *       activeTab: 'monitor',
- *       setActiveTab: (tab) => set({ activeTab: tab }),
- *   }));
- *
- * First migration target suggested in docs/ARCHITECTURE.md is the
- * `let isCreatingRoom = false` / `createRoomTimeout` singleton pair in
- * NavigatorRoomCreatorView.tsx — a ~5-line conversion that removes a
- * react-compiler/react-compiler "writing outside component" violation.
+ * Do NOT pull the whole store (`const all = useStore()`) — that
+ * subscribes to every change and defeats the point.
  */
-
-export const createNitroStore = (): never =>
-{
-    throw new Error('createNitroStore is not enabled. See docs/ARCHITECTURE.md proposal #5.');
-};
+export { create as createNitroStore } from 'zustand';