perf(board): deduplicate board fetch with React cache()

Wrap getInitialBoardAsync in React cache() so the same board data is
fetched once per request instead of 3 times (layout, page, metadata).
This is the same pattern used for auth() in the codebase.

apps/nextjs/src/app/[locale]/boards/(content)/(home)/_definition.ts

@@ -1,9 +1,11 @@
+import { cache } from "react";
+
 import { api } from "@homarr/api/server";
 
 import { createBoardContentPage } from "../_creator";
 
+const getHomeBoardAsync = cache(() => api.board.getHomeBoard());
+
 export default createBoardContentPage<{ locale: string }>({
-  async getInitialBoardAsync() {
-    return await api.board.getHomeBoard();
-  },
+  getInitialBoardAsync: getHomeBoardAsync,
 });

apps/nextjs/src/app/[locale]/boards/(content)/[name]/(board)/_definition.tsx

@@ -1,9 +1,11 @@
+import { cache } from "react";
+
 import { api } from "@homarr/api/server";
 
 import { createBoardContentPage } from "../../_creator";
 
+const getBoardByNameAsync = cache((name: string) => api.board.getBoardByName({ name }));
+
 export default createBoardContentPage<{ locale: string; name: string }>({
-  async getInitialBoardAsync({ name }) {
-    return await api.board.getBoardByName({ name });
-  },
+  getInitialBoardAsync: ({ name }) => getBoardByNameAsync(name),
 });