Add initial project files

docs/architecture/react-frontend/layers/refdata.md

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+# Reference Data Layer
+
+App-wide API-backed lookup datasets reused across modules — countries, languages, currencies, time zones, common option dictionaries. The layer fetches them, exposes reusable query hooks, and owns their cache keys.
+
+Small dataset → one file; larger → a folder:
+
+```plaintext
+refdata/
+├── countries.ts
+├── languages.ts
+└── currencies/
+    ├── api.ts
+    ├── dto.ts
+    ├── mapper.ts
+    ├── queries.ts
+    └── keys.ts
+```
+
+## Rules
+
+1. App-wide datasets only — a dataset owned by one feature belongs in that module's `data/` layer.
+2. May use `#infrastructure/axios` and `#infrastructure/reactQuery`; must not import from `view/`, `pages/`, or feature modules.
+3. Expose hooks or small service functions — consumers should not know endpoints or cache keys.
+4. Add mappers when backend DTOs differ from app-facing data, tested like module data mappers.
+
+## Example
+
+```ts
+import { keepPreviousData, useQuery } from "@tanstack/react-query";
+import { axiosClient } from "#infrastructure/axios";
+import { createKeyBuilder } from "#infrastructure/reactQuery";
+
+export const countriesCacheKey = createKeyBuilder(
+  "REF_DATA",
+  "COUNTRIES",
+  "LIST",
+);
+
+interface CountryData {
+  code: string;
+  name: string;
+}
+
+export const useCountries = () => {
+  return useQuery({
+    queryKey: countriesCacheKey.store(),
+    placeholderData: keepPreviousData,
+    queryFn: async () => {
+      return axiosClient
+        .get<Array<CountryData>>("/datasets/countries")
+        .then((data) => data.data);
+    },
+  });
+};
+```