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

yarn.lock

@@ -3221,3 +3221,8 @@ zod@^3.22.4:
   version "4.4.3"
   resolved "https://registry.yarnpkg.com/zod/-/zod-4.4.3.tgz#b680f172885d18bbebf21a834ea25e55a1bbf356"
   integrity sha512-ytENFjIJFl2UwYglde2jchW2Hwm4GJFLDiSXWdTrJQBIN9Fcyp7n4DhxJEiWNAJMV1/BqWfW/kkg71UDcHJyTQ==
+
+zustand@^5.0.13:
+  version "5.0.13"
+  resolved "https://registry.yarnpkg.com/zustand/-/zustand-5.0.13.tgz#06995c126e8903cd27100af04da91c36ae3051ed"
+  integrity sha512-efI2tVaVQPqtOh114loML/Z80Y4NP3yc+Ff0fYiZJPauNeWZeIp/bRFD7I9bfmCOYBh/PHxlglQ9+wvlwnPikQ==